Despite being young, I still work in old money, as my projects are very 'military'
I usually go around 2" for a partial bag, and now 3" for full, though be careful packing as it can bulge out even wider.
If you make the sides bigger and have the Velcro taught, you can get away with that width with little rub, but a slimmer 2" can be easier to Velcro on, bearing in mind what you intend to pack
If you want to be a bit clever, you can taper the bag, 2" wide at the cranks and 3 3/4 " at the front, alot of room, no crank rub and works perfect with a rolltop
Also adding dividers has an effect on width, a 2" bag with a 2" spacer sewn in keeps a taught shape with little bulge but ofcourse less storage
Do you intend on having Velcro sewn directly into the seam? Or a webbing ladder similar to alpkit, this also effects fit quite a bit in practice
