I'm not sure how Merlin would have us select a unit without having to measuring it. Can Merlin answer "how long is a foot?" without appealing to other units, which in turn raise the same question toward themselves?
See, even if I arbitrarily say an inch is "yay" big, I'm still suck having to "measure" that distance in some way, and it would appear that under Merlin's view, I would have to appeal to some other unit when undertaking the inch's measurement -- since measure entails unit. And of course, I'll have the same problem with that sub-unit.