i'm making a cake for about 20-25 people...what size cake do i need to make?? i want to make a two tier round cake but i didn't know how many inches on the bottom and how many on top???

http://www.wilton.com/wedding/wedding-cakes/wedding-cake-data.cfm
The above chart is what I use to determine servings.

The smallest round cakes are 6/8. 6" serves 12 and the 8" serves 24, so you're looking at 36 wedding servings with the smallest cakes.

Of course people are happy with more cake, so you have lots of wiggle room to cut them bigger!