A doorway in general? I would hesitate to generalise on door sizes for TF2. I presume you're talking about the dynamic doors used for spawns. Spawn doors and round gates will be your only dictated door sizes; and even then you can improvise. Pretty much every doorway is different, especially across maps.
"Rooms" in 2fort, well, badlands, goldrush and Dustbowl tend to be 256 high or less. Other maps tend not to feature "rooms" but large warehouses or odd shapes designed to influence gameplay (Badwater does this a lot, and Hydro), something that would probably not constitute a "room" in the contexts used here (Other than the unique case that is Badwater). But that's just me being pedantic.
Technically. No room abides by any particular rule other than give the players enough room to fight it out, but not so much room that it takes ages to get anywhere (a common ammetuer mistake as players tend to choose scout to explore their maps quickly, when testing maps a player should utilise a soldier for a good idea on map lengths). Size will depend on significance of the location (possible objective? which would benefit from being larger than usual, with a combination of entryway sizes) or dictated by a detail theme.
If you simply can't make your mind up, revert to a model for an answer (be it practical or inspirational).