I'm doing a dummy for a bridal fair Tuesday night and need DH to cut the board for me today. Cake size will be 6, 9, 12...so what size should my board be? I'm thinking 14 but DH is saying 15 - to keep the 3" size difference going, does that make sense? 
Any advice/insight is appreciated! TIA! 
A lot depends on how it's going to be decorated, but I think your DH is right, 15 if not 16. I wouldn't do 14, that gives you only an inch around and that won't look right.
You could cut sample boards out of large paper (newspaper, or posterboard), then stack your pans upside down on each size to see what looks best to you.
It may not look exactly like the cake, but will give you an idea of the proportion at least . . .
Laura.
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