Depends on what you're looking to do. If you ever want to go to print then you'll need at least 150-300dpi. (Depending on the medium and the color content.) As far as physical size/dimensions goes, the larger the better -- you can always downsize later, but you can rarely upscale without quality becoming an issue.

For a piece going to print, I'll usually work at 300 (+) dpi and dimension of [i]at least[/i] 4,000px X 4,000px. (Even for smaller-sized products. It makes it easy to paint all the little details in and you never have to worry about quality when you work large.)