Unless you happen to own a micrometer, the best suggestion that I have is to take them to your local card shop (if you have one) because I am sure that they have many different sizes laying around and see if you can test them to see what sizes you need.
This is a good question, that I'm sure a lot of people have. I'll try to come up with some kind of "tutorial" on how to determine it at some point. There is 1 size that I use more of than anything else, too, but I can't remember off the top of my head which it is. When I get home this evening I'll be able to let you know.