Hmm, that sort of depends on a couple of factors. Memory being the main one. If you have enough memory you can render 10,000 x 10,000. The problem is photoshop file format requires a lot of memory. If you need super high res images you can have an unlimited texture size by use maya's bot format, if rendering in maya software render, or memory mapped textures for mental ray. You would have to flatten the photoshop image into a standard tga or tiff then convert the flattened image into a bot or map. makebot is a mel command that will make bot files from texture image files, and there is a comparable command line command for mental ray but I don't remember it off hand.
As for rendering output image size restrictions I'm not sure the exact size limit of an image rendered out of maya but from what I remember on forums its something like 40,000 x 40,000, barring you have the memory to support such a resolution. Can anyone correct me if I'm wrong.