when making a tier cake waht size pan should i use to feed 50 people?

How many tiers - 2, 3 or 4?
Check out the Wilton wedding cake cutting guide - one of several guides that will help you determine how many and what sizes to use.

This is a good site for playing with cake sizes: http://shinymetalobjects.net/cake/calculator/cakulator.cgi?option=2&option=Select+Cakulator

Kim

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