What size should I make a smash cake for a photo shoot (with the baby).  I'm trying to decide diameter and height.  Any advice appreciated!

Usually they are 4-6 inches in diameter and 2-3 inches high. Depends a lot on what size pans you have and what kind of decorations. I think 4 inch is more than big enough if you have that size. It also fits well on a high chair tray. Some people go with a cup cake.

I've done a few of these with a professional photographer.  She usually prefers 6 inch cakes, 4 inches high.  It's a bit overkill for actual cake consumed, but looks good in the photos, lol.  

I always do 6", 2 layers.  And NO FONDANT decorations for the little darling to choke on!