I'm baking a cake that has a fairly large (to me) topper - 5 inches wide by 8 inches tall. How large should the top tier of the cake be so that it will not be overwhelmed by the topper?
That sounds like a personal preference! What you could do, if you have the topper - is stack your cake pans and put the topper on top and see how it looks.... Personally I wouldn't design the cake over the topper - just make sure the top tier has room for it and it will be beautiful!