What is the optimum height for corridors?

128 is good. But a texture with it's default size of 0.25 is as big as a 256x256 wall. Better always keep the sizes with a power of two (2, 4, 8, 16, 32, 64, 128, 256, 512, ...).

DasMatze

nope matze, as I've learnt though trial and error (mainly error) 128 is bad, it leaves no room for jumping or standing on things I'd say 144 as a minimum-ish with 256 being a maxium (max is far more flexible because you can have high ceilings far more realistically than low ceilings)

i like 192

I mean obviously you can go to 128 if you must, it just about works but don't make all of your corridors 128, and as matze said anything that is a multiple of 16 generally, if you really need to fine tune it further (can't see why you would need to though) to the nearest 8 will do, 4 or 2 is just pointlessly accurate.

I always use the door frame prop I'm using as a base, keeping the hallway inside slightly higher, and adjusting inside based on realism for the environment. RED wooden buildings will have higher cielings with framing and loft spaces, while BLU industrial buildings will have lower cielings in hallways but have more open "workspaces" with metal rafters...

Mainly, I just eye it in game and see what looks right.

For me, I vary the hallyway heights. Optimun is relative as you don't want to cookie cut your maps with the same sized areas over and over (Might be more forgivable on a CTF or 5 point CP map).

For me it is just as important to pay attention to the width of the hallway as well. Take 2Fort for example- the widths and heights from hallway to hallway change (sewers, to main hallway, to narrow long hallway, spiral corridor, to main stairs leading to intel, Intel main hallways, to narrower hallways leading directly to intel room, ect)