Whenever you need to re-size something, here's the easy math to do it:
(Finished size you need / Finished size you have) * 100
Example:
Need larger copy:
You have a 5" design, but need to enlarge it to an 8" design. (8/5) * 100 = 160%. If you put your 5" design on a copier and enlarge it 160%, you should end up with an 8" final design.
Need smaller copy:
You have an 8" design, but need to reduce it to be a 5" design (5/8) * 100 = 62.5%. Put your 8" design on the copier and hit reduce by 62.5% to get your 5" design.