The 3ft version should be a very nice size, not too big and manageable but still leaving scope for faithful internal and external detail. I have to say that at Brighton Modelworld, I have always admired the exhibition of dolls houses and this sort of modelling would be quite similar in many respects.
As has been said earlier Paul, don't make it too boxy, it's the combination of slight boxiness and curves that give these boats their charm.
Do you have to be a vintage gentleman to build one though – in which case I think many of us might qualify….
Colin