My question is less stupid than the title sounds.

What I'm wondering is how to know what size I should make my textures. IIRC textures have to be sizes that are divisible by 64? I think I understand the concept of UV unwrapping, but there's no way to do that in radiant (right?), so how can I make textures that are the correct size for certain brushes?

You can use the Set command in the surface inspector. Just assign the number of values for the texture to to repeated in the X and Y axis. You can also stretch and shift the textures.

There is also the FIT option in the surface inspector which makes your texture in question be stretched to fit the surface its on.

Texture sizes need to be powers of 2.

2, 4, 8, 16, 32, 64, 128, 256, 512, etc

for the most part.