I have to make a rectangle cake for 50 people - According to Wilton's guide Ihould use the 11x14(or 15)) pan. I baked one layer of it tonight and it doesn't look very big. Is this the size you would use for 50 people?

A single-layer 12x18 would work, or a double-layer 9x13...
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

It should be rather simple to figure out how many servings one can get from a rectangle cake. If it is 2" tall it is cut into 2x2x2 sq so divide the dementions by 2 thus you 11x14 should give 35 servings.
If you make that same size 4" tall then the serving size would be 1x2x4, so there would be 70.

kakeladi... i'm slow at math! how do you divide the dimensions by 2? Which numbers?