I find on a bridge hammock, narrower underquilts work better. That is one reason that the Mt. Washington line of quilts are only 42" wide. Some of the underquilts that Grizz makes for his Grizzbridges are only 36" wide. So I say pick a number between those 2 and you should be fine width wise for the BMBH. As for length, that is entirely up to you.