If you are just going up a size and it's a multi size pattern you should be ablt to use the size difference between the other sizes to gage how much to add on.
Good Luck,
Susan

It depends a bit on the type of pattern, and what part doesn't fit. Does it need to be bigger all over. Is it for a child or an adult, etc.

I found a helpful hint on page 4 of the fit brochure on the Simplicity link, about enlarging a dart...

http://www.simplicity.com/index.cfm?page=section/fittips/fitbrochure/reallyknow.htm

also, more info here:

http://www.sewnews.com/library/sewnews/library/aatips21.htm

http://www.sewnews.com/library/sewnews/library/aatips16.htm

good luck!