Bigger is better. Rectangular vs. square doesn't matter, especially if the textures are seamless. Bump maps are great. I'm most likely to buy seamless nature textures - tree bark, ground cover, leaves, snow, etc. I've also bought some fabric and lace textures. I've been happy with most of the textures I've bought. I was disappointed in one pack of fabric textures; all the textures were the same, except different (solid) colors. I was hoping for more variety.