My current version is ~50", but I've also made a 60" one. My take is that, with winter insulation, I tend to keep my body tucked in anyway (to stay warmer) and don't need the extra width. Even the 50" is wider than I really need. For summer you want to spread out to cool off, and get more comfortable in the process. I think I'll keep using the narrow one when it's cold and switch to the wide one for summer. I also like the weight reduction of the narrow one when I'm carrying a bunch of insulation in the winter, whereas the extra hammock weight won't bother me as much without having to carry quilts.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson