What is the max resolution a texturepack can have without installing any other mods like Optifine?
512x512 resolution is the highest. It can be pretty insane though. 256x256 is more common. Here's a planet minecraft page about it, I think they have some good opinions on them. LINK(warning: link doesn't provide information specifically on 512x512, only up to 256x256, but it does talk about above 256x256)