I used to stick to the 64 grid, but I've recently gone down to 32 because I want a bit more granularity. Working on a 16 unit grid feels off because I stop being able to eyeball the differences. The difference between 160 and 176 is pretty insignificant, while I think the difference between 160 and 192 is worth considering.
64 unit jumps require crouch jumping which doesn't feel great. 32 unit differences require a jump, though they don't always look like they should and are irritating for it (though they're not as painful as 64). 48 is the magic height difference cause it feels jumpable and
is jumpable, though I usually try to only use these where a floor height will be off the 32 unit grid for as little as possible. I believe the train cars at Badlands mid are 48 units tall (with 48 unit crates)?
96 is a nice height difference (esp. for scouts), though I think I prefer 128 for anything someone might hide behind cause it feels less like your head might be poking above for an enemy to notice. 192 is a nice height difference, though I've also found myself using 160 or 128 in similar cases (a 16 unit floor slab will allow for a comfortable route under provided the space isn't very large, and sometimes you do want a space to feel small).
I make my walls 16 units thick, which feels about right for concrete. Buildings with small framing and thin walls (wood and metal shacks) get 8 units of wall thickness and 8 units for the interior beams. I'll sometimes use 32 unit thick walls if it's convenient.
I generally use 128x128 or 128x160 for flanks, 192x128 for decently popular routes, and just make something up for anything wider (though usually these are at least 160 tall cause anything less feels cramped). Taller doors usually coincide with areas where a height advantage looks down onto the door so that the top of the door doesn't get in the way of fighting.
Room size is a really inexact science and I don't have any advice beyond "Do what feels right." Height differences have more exact answers about what you should use when cause they're about individual gameplay moments between two players, while route sizes need to accommodate a lot more variables. I think generally route length corresponds with route width - long thin flanks really suck.
Most wall textures are 192 units tall, I believe.
Assuming a 0.25 texture scale, most HL2 textures are 128x, most TF2 textures are 256x. A select few are larger than that (brick/wall028 is 256 wide, 512 tall). No texture is 192 units tall unless you scale it that way yourself (or the texture uses $basetexturetransform).