Board size
I think it is best to use a board you like in flat water first. It is a good place to get used to it before you try it in the waves. Be sure to feel how it carves through gybes, how quick it gets planning and how much you can float on it.
Most importantly use a board the is big enough to float on when the wind drops, mainly because it is usually gusty in down-the-line conditions and the extra volume will help you to get over the white-water on the inside.