Very quick question,
What is the smallest top tier size would you go??

I'm doing a wedding cake for 200, They want square. What would look the best? I know what each cake yields but what size difference between each tier is what I'm after also!

6" is typical.

I prefer 4" difference between tier sizes, when possible.

6" is traditional for the top tier. It's what commercial toppers are sized for and what the florist will expect if they're the ones doing a topper.

For round tiers I love the look of a 3" difference; for squares, I think a 4" difference looks best.

Mine usually end up as 2" difference and look fine. I've done both 6" and 4" top tiers that look fine. Of course I was the one making the toppers if there was one. So if they're having a florist do it, either somehow coordinate that or go with a 6". That's what I would do anyway.