I'm making a tiered Vegas cake with a single die as the top tier. This will be my first time mixing square cakes with rounds, and I'm not sure what sizes I should use. So, let's say I use a 6" square pan-- how much larger should my next round be for the square to fit on it with ample room on the sides of it?

You can stack your pans to get an idea of how it will look

I could, if I had them. I have to go buy a square pan since I don't own a set yet, so I'm trying to figure out what I need to go get!

Why not try cutting a 6" square from paper and place it on top of your round pans or round circles until you find the right combination that you like?

I would say you also need to take into consideration how many servings you need and how many tiers you want total.