In reply to jk90128: Just had a look at the brilliant little "Mont Blanc 4808m - 5 Routes to the Summit" guidebook and Francois Damilano recommends a
50m (minimum) single rope
I'm afraid I can't remember what length or thickness I used on the 3 Monts route, and I wouldn't want to disagree with such an authority.
That said, given the altitude and length of the routes, I'd always prefer the lightest one I could get away with, so in my case, ideally thinner than 9.8 or 10mm. However the ease of taking coils obviously varies; skinny sport climbers (like me) can find it slightly more awkward than barrel chested giants for the same length of rope.