this is a loose standard of what some people prefer to mat by:

for use mat size

4x5 and 5x7 11x14 or 13x15

8x10 14x17 or 14x18

11x14 16x20

16x20 20x24 or 22x28

now these are not hard and fast, just ones that some galleries prefer. Like ann said, it all comes down to personal taste of what you want. Just remember if it is for a specific gallery or museum, ask what they prefer.