I was wanting to alter the size too, and did some math (assisted by my next door neighbour) and we worked out how to make a quilt that is 43.36" wide x 72" long as I sew for Aussie hero Quilts and their specifications are 42" x 72". I've popped a tutorial on my blog if anyone is interested in seeing how we calculated it out.
http://beeshebags.blogspot.com.au/20...elly-roll.html
Hugs and hope my tutorial helps you with your measurement dilemma a bit.
Naomi in Oz