Cakes look best with a 3" or 4" difference in size. You can do a 2" difference in size, but IMHO that doesn't give enough definition or enough deco room. Never do just 1" difference in size unless you are going to carve it down into a mountain! 
6"-9"-12" and 6"-10"-14" are common tier sizes. You can do 6"-8"-10", but like I said above, I prefer the larger size differences.
HTH!
Yup, makes sense. 6-9 will look great, but only gives you 44 servings (using the wilton guide, which is pretty much the industry standard). 6-10 will give you exactly 50 servings using the wilton guide. If you want larger slices of cake, then you will need to go even bigger. Personally, I'd make an 8-12 or a 9-12, but then my friends and family tend to eat a lot of cake.
Quote by @%username% on %date%
%body%