Serious question here, as I have a five-foot tall wife and am planning a circ:
At 15, your son is about as big and possibly as strong as he will ever get. Do you have special aids to help him do his watches, mechanical or simply something to accommodate his stature?
I would've had a 45 footer, but kept it to 40 because my wife, while fit and relatively young, could not handle the mechanical forces on a 45 footer without electrical
or other assistance, and realistically, passagemaking means you can't call the off-watch if you can't work the boat.
At 40 feet, she can handle everything up to 30 knots. At that point, it would likely be "all hands" , anyway.