I would advice you to just go and have a look at the detail on some of the props in the game and make your own judgment.
A lot of the small props (around and below 64 units large) have the lowest resolution textures in the world. It's amazing how terrible they are, but no one seems to notice, because it doesn't matter.
I grabbed these 2 images for me to look at, and remind myself while making the construction pack textures:
The conclusion I reached was that I needed to make everything as low-res as I could get away with.
For example, the box of nails
https://dl.dropboxusercontent.com/u/1281220/old/SS-2010.11.01-19.15.17.jpg is 128x128, comes with 3 sizes and 2 skins. And the total size for the vtfs and vmts is 12.0 kb. Because it's such a insignificant prop that no one looks at, no one remembers it exists it should be cheap to include. But it can sit in a corner and make the room feel less blocky just by being there.
The next step up to 256x256 would increase the size from 12kb to about 90kb. Doesn't seem like much but then again, does it matter? and if doesn't; why not use that space for 7 other 128x128 textured props?
Edit: also those hammers you see in the nailbox picture are 512x512 for some reason. I didn't make those.. (but I know who did and I'm doing the disappointed face at you!) that's almost 30 times the filesize of the nailbox yo. 30!