How to determine texture size for pixel perfect?

I am using 2DToolkit and building 1280X720 resolution game. To create pixel perfect cube objects I create texture size equal to the cube size in game. So if cube is 100X100, texture need to be 100X100 to be pixel perfect.

My question is: How would I determine the texture res size for a capsule or a sphere?

Thats not really how texturing works, if you zoom right in on your cube that would fill the 1280x720 viewport the texture would look really bad and low quality (as its resolution os only a paltry 100x100), it would only look pixel perfect if it occupied at all times only 100x100 pixels on the screen.

Also its a good idea to use textures that are a size of a power of 2 (eg 256x256, 512x256 etc), the larger the texture the nicer the quality.

Really you should just pick a texture size that looks right, not try to get accurate pixel dimensions using units of measurement.