Combined it's around a million adults. Of those, perhaps 500K-600K registered voters (many international/non-voting adults in DC especially, including embassies/staff etc). The answer, for those living there who seek representation/to get away (voting members) is as always to move outside of the federal district, something sane DC residents have done for centuries, at least back to George Washington, anyway.

But in truth as a policy/analytical reality (using a 'totality of circumstances' standard, if one will), their voices are quite strongly represented within the legislative branch, so I don't think it is quite the impact many leftists decry.
I'd again highlight how rich it is that this goes back to Democrats (illegally) excising Alexandria/Arlington to Virginia to protect slavery there, antebellum. Now, Democrats would be outraged to see them returned.
ETA: an EO cannot create a Senate/House seat, period.