What dimensions make a prism a fair die?

I was thinking about the Curiosity Box dice set and how the d5 is a triangular prism, the d7 is a pentagonal prism, and the d9 is a heptagonal prism, and it got me wondering what dimensions make a prism a fair die. If you're making a prism whose faces on the ends are regular n-gons, what height, relative to some dimension of the n-gon, would make the prism a fair die?