Hi, Claudia. I think the card size stampers use most often is A2 which is 4.25 by 5.5 inches. If you cut a sheet of 8.5 by 11 inch cardstock in half, it creates two perfectly sized pieces which you can score into two card bases. That is the size I make most of my cards with.
Maybe someone will have a chart for some of the other standard sizes. :)
Here's a handy PDF that's a web link for envelope and card sizes:
Thanks Lee for the good information. This will help me alot.