Seems you're going to get a lot of opinions with that question. I'll try to simplify. Longer=more bouyancy=easier to ride. However, more bouyancy=more difficult to get up on.
There are two styles of boards: surf and skim. Surf style are thicker, more bouyant and easier to keep in the pocket. Skim are almost as thin as a wakeboard, more difficult to keep in the pocket but significantly easier to get up on.
If your boat has a tiny wake, I would probably choose a long surf style board (5' or more). If you have a decent wake, get a skim style board (4'8" or longer). Your kids will appreciate the ease of getting up.