The bride wants 3 or 4 round tiers to serve 200. I am doing a separate cake for her anniversary, so the top tier can be used.

The 1st tier will be two cakes torted and filled. The second tier will be three full layers with bc between. The 3rd and possibly 4th will mimic the 1st.
(Does this make sense? I'm confusing myself at this point.)

What size should each round layer be to serve 200?

Thanking you very much for your capable assistance.

6/10/12/16 will serve 206 per the wilton chart: http://www.wilton.com/wedding/wedding-cakes/wedding-cake-data.cfm

Bear in mind that the tier that is 3 full layers should have a higher price per serving. Even tho' it's taller, you will get the same number of servings from it because it's the same surface area for cutting. And also bear in mind that this will be at least a 6" tall cake so when it's cut and served, if they are using standard 6" plates, the cake may lop over the edge of the plate (not an attractive way to serve it).

Anyway .... in that one piece of 6" cake, they are getting 50% more cake so the price should be 50% higher for that tier (i.e. if your regular price is $3/serving, then this tier would be $4.50/serving.

A 1x2x4 = 8 cubic inches. A 1x2x6 = 12 cubic inches. 50% more cake. 50% more money.

If she wants the bells of whistles of a taller tier, then she pays for it.