I am making a cake for 75-100 people at a birthday party.

What size round tiers.

Would 12, 9, and 6 work, or is that too much difference in size?

Hi...I haven't made one those sizes so can't say from experience but I think it will look fine as I've seen 3 inch difference tiers in the galleries all the time.

Also, did you want party size servings? If so then you would have about 68 servings of 1.5x2x4. If you're planning smaller servings or wedding size then you'll end up with just over 100 servings of 1x2x4.

Hope this helps...if not, maybe this will give you a bump so someone else with more experience can come by and give a better answer.