I had the impression for the longest time that most of HL2's main level textures were 1024x1024, but when I look through the textures in Hammer, it says most of them are only 512x512.
I'm pretty sure most engines that have "power of 2 textures" can go pretty much up to any tex size, as long as it's a power of 2... at least in the Quake and Unreal engines.