Pipe clamps (sometimes called bar clamps) work by attaching the clamp mechanisms onto a standard size pipe. You can attach the clamp mechanism onto any length of pipe to create clamps of almost any size. But unless you have a huge shop, you'll need to pick some sizes. Too small is no good, and neither is too big since when the clamps get too big they tend to be hard to work with, and the weight of the pipe can tips things over if it's unsupported.

A nice way around this problem is use pipe couplings. The couplings are cheap, usually less than $1, and they allow you to connect pipes together to make increasingly larger clamps. I buy all 2-foot length pipe and then use couplings to make the size of clamp that I need as I need larger clamps.