I think that design looks lovely in any number of tiers and sizes.
Ok, problem seems to be solved! Just talked to the wedding planner and confirmed that the cake topper is 7 inches across and has pins to go into the cake in the middle of the topper (not the sides, like I feared). So, I'm making 6, 10 and 14 inch tiers which will feed 128 people. The other tier will be a kitchen cake :) This way, the display cake works out exactly how it needs to with the wood effects, and I can still accommodate extra servings.
whew..... now off to bake!
Thanks for your input and help, everyone!
I see that this was last year but for future reference if you put your cake logistics into the this app "cake slicer" http://cakeslicer.bakingit.com it will tell me how many slices you can get out of your cake depending on portion sizes. That way you can let whoever is cutting the cake know how large to cut the servings and show them the guide for cutting. I use this site all the time so that customers can see the logistics of their cake.