I'd go around 14' or so. Sort of depends on what you're going to be doing, and how you're going to do it.
Non-trailered, no activities (aside from boating), no friends along would allow you to go smaller (though I wouldn't want to be manhandling a 25 4-stroke very often.)
If a trailer is OK, if you plan to do a lot of fishing, camping, or anything requiring toting of gear, or if you think you or your kids might want friends along on occasion, or you plan to do long runs I'd opt for the larger size. Even if you can get everything situated in the boat, you still want a little room to manipulate the stuff and be able to have people move around.
A 25 will do OK on a 14, but that's about as low as I would go (though I have seen some driven by a 15; OK if you don't mind taking time to get anywhere.) If you go with a larger boat (15 or so or better), you should probably up the hp.