to feed approximately 100 people
If you use the wilton serving chart a 6", 9", 12" will feed 100 I believe.
I am doing a cake to feed 150 and I am doing a 8,12,16 - squares, with the center tier being a dummy cake. The extra cake is in the grooms cake which is an xbox guitar hero guitar (carved from an 11x15 sheet).
Hope these dimentions give you some idea.
Here are a couple of charts. It depends on how big you want your pieces to be.
heres a link to the wilton chart, which is what i go by.