I couldnt tell you exactly what the tiered sizes are...but I have a suggestion for you. How about you ask how many people are being served and then go from there, you could pretty much figure out the tiers on your own from knowing the amount of people being served at the reception. Then you could do this same exact design with an accurate count on servings.
Hope that helps and good luck
My guess is 4" square, 8" round, and 10" square (with corners trimmed off)
Each tier is (2) 2" layers
6" square, 10" round and 12" square with corners trimmed
Each tier would be (3) 2" layers.
More likely it is the 4" tall tiers, because 6" tall tiers are harder to server and usually won't fit on a dessert plate very well.
I was thinking 6" square, 8" round and 12" square w/corners trimmed.