Well around 1k for the buildings, but we will try and find out what is the best optimized result
I also have a table of textures sizes = Vram usage.
The udk provide it in the doc files... It's very handy for calculating how much video will you need.

I'm makin` them in 2k. And smaller them later.
But i find out that high res painting then lower them keeps amazingly lots of the details that you painted in the high res. So i'm thinkin in jumping in 4k for first coloring then lower the texture for the ingame purpose.

Cheers, ColdWave