Originally Posted by
ulillillia
For supporting 800x600 resolution, anything wider than 600 is bound to cause problems. 640x480 is extremely rare nowadays, mainly only used during safe mode or Windows installation. Heightwise, this isn't of concern for normal web usage, but for forum signatures, something like 64 would be a nice cap. That's about 4 lines' worth for Tahoma size 10. Thus, I'm suggesting 512x64 as the maximum (a texture size for 3D games).
For avatars, I'd suggest 64x64. Some forums use 80x80 and a few go into triple-digits. Here, since this isn't an artwork forum, 64x64 seems decent. 128x64 may also be okay for the extreme upper end.