Resizing DC (has been done before) and making a new state would also be within the rules of the Constitution.
How does declaring statehood change taxes...D.C. sets its own taxes. Most of the expensive real estate in D.C. is owned by the government or universities who pay no taxes. The rest of the residents have to make up for that to cover the city budget. Not sure becoming a state suddenly makes taxes drop. Taxes are a major gripe of D.C. residents a well as crime and shitty public schools in NE and SE which should never have been part of D.C. to begin with. Statehood simply is a political result for more seats but taxes cannot come due to the high cost of running the District which 50-60% is simply neighborhoods with no connection to the federal government.