Cab, I put a Bison 10" 4 jaw on my Clausing 1300 (13" swing) and glad I did. With a 15" swing, if I was planning on buying only one 4 jaw, I would not go smaller than 10" and would consider a 12" according to weight & whether I thought I alone could handle it. At some point, weight may determine whether you'll need one of those "over-arm" things I can't think of the name of, to suspend the chuck from to mount & dismount it.
I kind of go on the theory that bigger lathes draw bigger work and bigger work needs a bigger chuck.