XP- How big for a signature mat? (to replace a guest book that everyone signs)

lalaith50

I thought you lovely ladies might be able to help me--
We'll have about 200 guests, probably not all of them will sign. We're going to put one of our engagement pictures in the middle, which we could print up to 8x10, or obviously it could be smaller. I don't mind if it ends up being a huge, poster-sized frame and huge mat. 
What would be good? 8x10 picture in an 18x20 frame?5x7 photo in a 21x21 frame (like this http://www.bedbathandbeyond.com/product.asp?SKU=18072084 ?)other ideas?
Anyone ever done this before? (how big was everything, and how many guests did you have, and how much white room left over?)