What diameter rebar should I use in a 4’x3’x2’ sculpture?

You do realize that you only need rebar when you encounter tension forces, right?

Basically, any part that would flex under its own weight.

If this means nothing to you get 6″x6″ wire mesh (or something similar) and place it 1″-3″ below the surface of the sculpture. It is far more important in thinner areas than thicker ones. Any portion thicker than 12″ in cross-section, for sculpture as you’ve described, probably doesn’t need reinforcement.

This general assumes you’re working with cement, concrete or plaster.