Suppose that, for whatever reason you wish (different Compromise, failed American Revolution, etc) that Washington, D.C. is never built/laied out? What do we find in it's place? Does Alexandria take its place as the largest city there? Bethesda? Georgetown? Also, how reduced would such populations be, since there would be national capital for people to live and work in?
Not Bethesda for starters. It's pure suburb. Take out NIH and all those rich congressmen and lawyers and you've got just another horse town.
At first I'd say Alexandria, as Georgetown doesn't have much of a harbor. However I think Georgetown is situated on a better grade to access the river in terms of water mills, if only just a little bit and not for very long.
We'll see moderate development of both towns, but not even the amount of industry that existed there OTL. If they try to build a canal to the Ohio river it probably gets off the ground even later and even less of it gets done, reducing the site's convenience as a destination for foodstuffs and raw materials. If they can tap the rapids to the north, that's a significant boost to industry, but development is stalled until it's easier to transport goods to that area than Baltimore or Norfolk.
Plus the Potomac is a bad vector for accessing the interior. It's not navigable even inside the boundaries of DC. Going east or north you just run into the mountains. If a ship lands at Baltimore instead, you've got access to the National Road, and even if that's not built, you're still at the proper latitude to travel west and access the SW/NE passes (which means it's likely someone will build the road.) Baltimore also gets you closer to Pittsburgh and real access to the interior.
I'm guessing the twin towns never develop much above your average New England mill town. Combined metro area, maybe topping out mid-20th century at 150-200,000.