Personally, if I were designing a game today would use the following texture sizes for a good balance of nice visuals vs. level load time:
Ground level segments that are featured predominantly : 1024
Higher segments that player can't get close to: 512
Ceilings: 512
Player HUD weapons: 2048
Enemies: 1024
Boss or large enemies: 2048
Small entities (cups, table items, etc): 256
Medium entities (trash cans, etc): 512
Large entities (chain link fences, dumpsters): 1024
Very large entities (houses, etc.): 2048
"bond1 - You see this name, you think dirty."
SPECS: Windows7 x64, Intel Core i7 920, Geforce GTX 580, 6GB RAM