There is pretty much no relation between baren size and print size - in the sense that we certainly don't use 'small barens' for small prints, and 'large barens' for large prints.
In fact, it can be the other way around. One of the strongest barens in my collection is a Murasaki baren from Goto-san, and it is nearly a centimetre smaller than the traditional size. This helps make it even more powerful, which was a big help to me when printing the background to the
scroll print I made a couple of years ago - which was pretty large.
Whichever baren you are using, nearly all the power is applied in the 'active region' right under the ball of your hand, and the rest of the disk really doesn't press the paper much, if at all. Having a larger baren certainly helps to keep it horizontal in use; one that is too small would perhaps 'wobble' more in the hands of an inexperienced user.
