not sure how to find what size sheet cake to make for about 40-50 ppl. Can someone please help or direct me to the right place? Thanks! 

A great start is to search the web for "sheet cake serving size chart".  Those are based on the size of each serving. The wedding cake slices are cut half the size of party cake slices, and I believe it's because most wedding cakes are going to be double- or triple-layer (at least 3-4 inches in height) so maybe cutting a 1x2 inch slice is the same as cutting a 2x2 or 3x2 of a single layer party cake.

From my own experience at office parties and social gatherings, a half sheet (single layer) cake usually gets cut into about 24 slices and a full sheet will serve around 48.

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm