I am making a 6 tiered wedding cake. The bride would like alternating round and square layers. Could someone tell me what size pans I should use. I would like to start with the top being a 6" round.

How many servings do you need?

When somebody requests alternating shaped tiers I take my dummies and stack them to make sure the sizes work together. I would definitely recommend you do that.

But first things first, find out how many servings she needs and that will help you decide what size tiers to make.

Here are the right angle measurements for squares:
4" 5.66
5" 7.07
6" 8.49
7" 9.9
8" 11.31
9" 12.73
10" 14.14
11" 15.56
12" 16.97
13" 18.38
14" 19.8
15" 21.21
16" 22.63
17" 24.04
18" 25.46

If you're starting with a 6" round, it will fit on a 7" square.
The 7" square needs 9.9 inches, so it will barely fit on a 10" round.
The 10" round fits on an 11" square.
The 11" square needs 15.56 inches, so it will fit on a 16" round.
The 16" round fits on a 17" square--I'd use an 18" square instead.
(I don't think it looks right for any size 18" or larger to be only 4" tall... I'd go with 6" tall on all larger tiers to prevent them from looking short and squatty in proportion to the rest of the cake.)
This will serve 12/24/38/60/100/& 162 (if 4" tall) or 243 (if 6" tall) to total 396 or 477 (depending on the height of the 18" square base tier).

I suppose you could do a smaller version, if necessary, by putting a 6" round on a 6" square, then 9" round, 10" square, 15" round, 16" square. This will serve 12/18/32/50/88/128 to total 328.

It's best to find out the serving goal first, of course.

Thank you both! I honestly don't think she is as concerned with the number of servings as she is the look! This information is very helpful!!