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!