Here is short summary of useful data for the States in the Union, sorted by claimed size (unusable mana zones’ contribution to area size is usually disputed).
Marches of the Montana
(Pre-Fall: Alaska/southwest, British Columbia/west shore, Washington, Idaho/north, Montana, Oregon/north)
Census: 980,000 sq. miles, pop. 239,500
Capital: New Benton
Independence State
(Pre-Fall: Indiana, Ohio, Pennsylvania/south, Illinois/southwest, Kentucky, West Virginia, Virginia/west tip, Arkansas/northeast, Tennessee, South Carolina/west tip)
Census: 728,000 sq. miles (+377,000 of eastern mana zone), pop. 727,000
Capital: Vernon
Union Capital: Nashville
Marches of the South
(Pre-Fall: Utah/south, Colorado/west border, Arizona/northeast border, New Mexico/northwest border)
Census: 646,500 sq. miles, pop. 489,500
Capital: Dolores
Dakota Marches
(Pre-Fall: Manitoba/southern border, North Dakota, Minnesota/west, South Dakota/north)
Census: 507,000 sq. miles, pop. 230,000
Capital: South Branch
Lake Marches
(Pre-Fall: Minnesota/southeast, Wisconsin, Michigan, Illinois/northeast, Ontario/south tip)
Census: 490,000 sq. miles, pop. 187,000
Capital: Lake Mills
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Marches of the Algonquin ⊗
(Pre-Fall: Quebec/south border, Ontario/southeast tip, New York, Vermont, New Hampshire, Maine, New Brunswick, Nova Scotia/disputed; Pennsylvania/north)
Census: 435,000 sq. miles, pop. 153,500
Capital: New Drum
Eagle Republic
(Pre-Fall: California/north, Nevada/west)
Census: 358,000 sq. miles, pop. 318,000
Capital: La Grange
Marches of the Star
(Pre-Fall: New Mexico/northeast border, Texas/northwest)
Census: 303,000 sq. miles, pop. 470,500
Capital: Seminole
Later State ⊗
(Pre-Fall: Kansas/east, Oklahoma/north, Missouri/southwest)
Census: 293,000 sq. miles, pop. 126,000
Capital: New Riley
Lone Star
(Pre-Fall: Texas/northeast, Oklahoma/south border)
Census: 270,000 sq. miles, pop. 159,000
Capital: Decatur
Cheyenne ⊗
(Pre-Fall: Wyoming/east, South Dakota/southwest, Nebraska/west)
Census: 242,000 sq. miles, pop. 357,500
Capital: Alcova
Winnebago State ⊗
(Pre-Fall: Nebraska/east, Iowa/west border)
Census: 240,500 sq. miles, pop. 310,000
Capital: Tekamah
Greater Idaho ⊗
(Pre-Fall: Idaho/south, Nevada/northeast, Utah/northwest)
Census: 184,500 sq. miles, pop. 283,500
Capital: New Boise
Fremont
(Pre-Fall: Colorado/east border, Kansas/west)
Census: 180,000 sq. miles, pop. 188,000
Capital: Fremont
True Colorado
(Pre-Fall: Colorado/north & east)
Census: 140,000 sq. miles, pop. 113,000
Capital: Bailey
True Missouri
(Pre-Fall: Iowa/southeast, Missouri/northeast)
Census: 137,000 sq. miles, pop. 90,000
Capital: Florida
Pacifica ⊗
(Pre-Fall: Oregon/south)
Census: 128,000 sq. miles, pop. 67,000
Capital: Bandon
Yellowstone ⊗
(Pre-Fall: Wyoming/west)
Census: 125,000 sq. miles, pop. 160,500
Capital: Colter Bay
Sunshine State ⊗
(Pre-Fall: Georgia/south, Florida/north)
Census: 121,000 sq. miles (+92,500 of mana zone at the tip), pop. 22,500
Capital: Madison
Bayou
(Pre-Fall: Louisiana/east, Mississippi/south border, Alabama/south border)
Census: 119,000 sq. miles, pop. 65,000
Capital: New Monterey
Atlanta
(Pre-Fall: Mississippi/northeast, Georgia/north)
Census: 105,000 sq. miles, pop. 52,500
Capital: New Atlanta
Lost State ⊗
(Pre-Fall: Arkansas/southeast, Mississippi/northwest, Alabama/north)
Census: 75,500 sq. miles (+151,000 sq. miles of uninhabitable mana zone), pop. 49,500
Capital: Tennessee
Missouri Protectorate
(Pre-Fall: Louisiana/north)
Census: 56,000 sq. miles, pop. 24,500
Capital: Farmerville
Crow State ⊗
(Pre-Fall: South Dakota/southeast, Iowa/north)
Census: 52,000 sq. miles, pop. 121,000
Capital: Custer
⊗ Denotes States that joined the Union of States after the initial signatories of the Treaty of The Union.
Changed populations in the Union:
Minotaurs (all central States; originally True Colorado): 8,500
Incubi (Sunshine State): 5,500
Thirsters (Marches of the Algonquin): 4,500
Dwarves (Fremont): 3,500-5,000 (estimate, no real census)
Dryads (Bayou): 3,000
Trolls (Sunshine State enclave; originally Caribbean islands): ~500
Mooneyed (Independence State): 200
Erlang (Eagle Republic enclave; originally Empire of Dàzhì): ~50
Neighboring Polities:
Northwest: Tribal Lands (alliance)
North: Hudson Bay Tribes
Northeast: New France
South: Aztlan & Sinaloa