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
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.