Territorial Evolution of the United States
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March 3, 1845 - Florida Territory is admitted as Florida, the 27th state. At ~66,000 square miles it is the 4th largest state; behind Michigan and similarly-sized Missouri and Virginia.
December 29, 1845 - The Republic of Texas is admitted as Texas, the 28th state. No precise measure of the state's area and claims is possible at the time, but even the most limited interpretation - the territory administered by the Republic and tacitly accepted by Mexico - makes it the largest state in the US.
June 15, 1846 - The Oregon Treaty sets the 49th parallel as the continental border between the United States and United Kingdom; the jointly-"held" Oregon Country is replaced to the south with an enormous expanse of unorganized American territory.
December 28, 1846 - Iowa Territory is divided; the southern portion gains admission as Iowa, the 29th state, while the remainder is left temporarily unorganized. It measures ~56,000 square miles, becoming the 8th largest state.
March 13, 1847 - Virginia regains ownership of the trans-Potomacan holdings of the District of Columbia, primarily including the city of Alexandria; this does not alter its ranking as the 4th largest state.
May 29, 1848 - The majority of Wisconsin Territory is admitted as Wisconsin, the 30th state. The remainder of the territory, though legally in limbo, continues to organize its civil government as a de facto continuation of Wisconsin Territory. The new state's area is ~65,000 - just shy that of Florida. At admission, it is the 6th largest state in the union.
July 4, 1848 - The United States of America annexes a huge proportion of the Estados Unidos Mexicanos, including Alta California, Santa Fe de Nuevo Mexico, and those portions of neighboring states claimed by Texas.
August 14, 1848 - Oregon Territory is organized between the Pacific Ocean, Alta California, the Continental Divide, and the United Kingdom. Though the territory is comparable in size to the state of Texas, there is neither expectation or desire that the region will be admitted as a single state.
February 13, 1849 - The "Honey War" between Iowa and Missouri is resolved by the Supreme Court of the United States, splitting the two claims roughly in half.
March 3, 1849 - The remains of Wisconsin Territory are combined with adjoining southern and western areas into the newly organized Minnesota Territory. Again, despite the model of Texas and the continuing failure to agree on a division of California, the Minnesota Territory's expansive borders are largely a matter of administrative needs; any future state is expected to be smaller. [1]
March 12, 1849 - A local government forms the State of Deseret, claiming most of Alta California as well as the portions of Santa Fe de Nuevo Mexico not claimed by Texas. The entity is not recognized by the United States government.
September 9, 1850 - Western Alta California is admitted as California, the 31st state. Even greatly reduced in scope from the Mexican state, it is easily the 2nd largest in the United States, spanning ~164,000 square miles.
The state's size is an accident. As it became clear that California would be free soil, compromise on dividing it into conventionally-sized states became impossible, as it would dramatically disrupt the balance between free and slave states (and thus the Senate).
East of "California" northern Alta California is organized as Utah Territory across the core of the declared "State of Deseret". The remainder of the Mexican territorial cession remains either nominally part of Texas, or unorganized.
December 9, 1850 - The United Kingdom cedes a small underwater rock in the Canadian waters of Lake Erie to the United States, for the purpose of building a lighthouse. America grows larger.
December 13, 1850 - The US government purchases Texas' westernmost claims. With adjacent territories, these are organized into New Mexico Territory, a region smaller than Oregon Territory or the reduced Texas, but still larger than California.
April 5, 1851 - Superceded by Utah Territory, the State of Deseret dissolves itself.
March 2, 1853 - Oregon Territory is split north-south. With the Columbia River serving as a natural starting point, the division is not even. The territory in the north (somewhere over a third of Oregon Territory) is organized as Washington Territory.
May 30, 1854 - America's unorganized territory is organized. In the north, between the Continental Divide, Iowa, and Minnesota Territory the sprawling Nebraska Territory is established. Kansas Territory is organized south of it, with most of the remainder being designated as Indian Territory. The exception is a narrow strip of land south of Kansas but excluded from Texas by the Missouri compromise; the area will gradually come to be known (colloquially) as No Man's Land [2].
This disposition represents a compromise between senators representing slave and free states in Congress. The Nebraska and Kansas territories will determine their free- or slave-soil status by popular vote, with the expectation that at least one might become a viable slave state, to be admitted as a pair alongside a northern free state (likely formed from either the Minnesota or Oregon territories). This compromise will not pan out quite as intended. [3]
The United States now holds 8 organized territories, 6 of which define at least some portion of their border by the Continental Divide.
June 30, 1854 - The United States purchases a large portion of the Mexican states of Sonora and Chihuahua in order to have a suitable buffer for a southern transcontinental railroad that would run primarily through slave states and territories to California. This territory will be assigned to New Mexico Territory on August 4.
January 11, 1855 - Boston Corner, Massachusetts is transferred to New York.
August 18, 1856 - The United States passes the prosaically-named [4] "Guano Islands Act" enabling private citizens to make claims on uninhabited islands on behalf of the US government, providing birds poop on the things in sufficient quantity so as to qualify as "deposits".
October 28, 1856 - The United States claims two small Pacific islands under the Guano Islands Act: Baker Island and Jarvis Island.
May 11, 1858 - Minnesota Territory is split east-west, with the eastern portion admitted as Minnesota, the 32nd state. The western portion becomes unorganized territory. The new state continues the trend of admitting ever-larger states - at ~87,000 square miles it takes Missouri's place as the 4th largest state.
Minnesota is also numerically distinctive in that it is the 17th "free state" in the union. As there are only 15 "slave states", its admission leaves power in the senate firmly in the hands of the free states.
August 31, 1858 - The United States claims Navassa Island - lying between Jamaica and Haiti's Tiburon Peninsula - under the Guano Islands Act.
December 3, 1858 - The United States claims Howland Island in the Pacific under the Guano Islands Act.
February 14, 1859 - After prolonged political wrangling, debate over the admission the 33rd state in the union comes to an end. Admission is not fought by representatives of slave states as strenuously as it might have been, because the avowedly free-soil territory has idiosyncratically elected two pro-slavery senators to first represent it. [5]
The state's borders are themselves the result of a similar mix of quid pro quo and factional calculation. Certain free state congressmen hope a large state will accelerate the decline of slave power in the House and Electoral College; certain slave state congressmen wish to set a precedent of larger states in the New Northwest to minimize free state representation in the Senate. [6] Through the resulting jumble of machinations Oregon's application for statehood fumbles to success.
Oregon is admitted as the 33rd state. Oregon Territory is divided on a line 34°W of the Washington Meridian [7], east of which the territory is combined with Nebraska Territory. The remainder is bordered to the west by the Pacific, to the south by California and Utah Territory, to the east by 34°W latitude and the Divide, and to the north by Washington Territory. At ~168,000 square miles, the newborn state just manages to seize California's brief title as 2nd largest state.
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[1] The kind of border shifting and clipping governments are comfortable with, and the kind AH mapmakers are comfortable with depicting, do not always correspond. If I was writing US history from scratch the constant merging and pruning would drive me crazy.
[2] Future tense intentional - the area was only infrequently known by that term in OTL, and this is before the POD, so it'll have to become popularized later. The alternative was to leave it - assume it remains an uncommon phrasing ITTL as well. Of course such a thing was completely out of the question.
[3] As the reader may have foreseen.
[4] Amirite folks?
[5] This part is OTL; history is weird.
[6] And there's your POD.
[7] OTL this is what we would recognize as Wyoming's western border.