I want to do a cake with similar proportions to the top two tiers of this cake:

http://beautifulcakepictures.com/white-flower-patterned-wedding-cake-with-black-topper/#.UWbMsEpzamg

But it needs to be bigger.  To feed about 40-50 people.  Any suggestions on tier size and how many layers?

ADo you mean the top double 6" & the 2 layer 8" or the 6", 8" spacer, 10"?

In any case those top 2 will serve more than you need.