What size do you need to choose? This depends.
If this is the only sweets you will be serving, you need a bigger cake than when you add other cakes and pastries to your catering.
We recomend the following:
 
7" Round = for ca. 16-20 people
8" =  for ca. 16-20 people
9"  = for ca. 25-30 people
10"  = for ca. 30-40 people
12"  = for ca. 40-50 people