Just a small note on the size of vessels. Just because medieval europeans prefered smaller vessels, it does not mean that they could not build larger ones.
Unfortunately I do not have the time right now to do some proper search and provide details. After some quick googling I found this interesting site regarding some huge vessels built during european
antiquity (with which I am more familiar), which I think is interesting even though probably off-topic.
http://www.answersingenesis.org/crea...2/i3/ships.asp
I'll revert when I have some more info on medieval ships.