I.8.xvii:
To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States,
IV.3.i:
nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
I want to increase the size though. Add Democrat areas to it, which would turn VA and MD red. Also, I would add a provision that says Congress cannot provide any financial aid or benefit to the New Columbia state unless the same benefit is provided to every other state on a per capita basis.