Hello
I'm going to bake a 9x13 cake for the first time, and I'm wanting to put a round cake on top, so my question is what size round will fit on top of a 9x13 cake. I'm a newbie at cake decorating and baking for that matter.

since 9 x 13

9 is the max size -- but that would look very out of proportion.

so some smaller size would be better visually

A 6 inch will work well, maybe even an 8 inch, depending on what you are planning on doing.