I'm making a 25th anniversary cake for a client and she wants a 12 inch square and an 8 inch square. I'm not sure how many people that would feed. I'm still not good about knowing that yet. Anybody know? 

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
 

Penguin22~~Welcome to the forum.  The link provided by CWR41is the one that is the "standard" for servings per cake in the USA. 

Keep in mind that even though it says it will "serve xxx number of guests", that is only IF it is cut properly.  Some people whack out great big slices.  People who have never cut a wedding cake  before (most of the people in the world.....), will have no idea HOW to get xxx number of servings from a cake.

I give my cakes away for free and if it is anything that is stacked or carved, etc., I provide a copy of instructions on HOW to cut the cake.  Here are a couple of wonderful links that will help you (or anyone) to get the appropriate number of servings from a cake.

http://cateritsimple.blogspot.com/search/label/cake%20comb

http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=142470&FTVAR_MSGDBTABLE=

http://larkcakeshop.com/CakeServeGuide2.pdf