Here's a quick tip that was suggested by April of Three Forks Cakes.
If you're trying to decide what size flowers would go well with a cake that you're doing, use silk flowers and cake dummies. Have flowers in a variety of sizes so that you can hold them up to the cake and see what looks right. That way you won't end up with flowers that aren't proportioned well for the size tiers you'll be using.
To elaborate on this idea, if you don't have flowers, you could always just crumple up some paper towels or tissue paper and see what size ball looks right on the cake.
Thanks for letting me steal your idea, April!