I think the important thing to do is have the jet pack correct in scale, not overall size, if you don't fall within those dimensions. By that I mean make sure that the body, thrusters, rocket, etc. are all made to the same scale as the rest of the jet pack. If a standard jet pack is 15" wide (that isn't true by the way, just an example), it may look way too small on a big guy. If you decided then to make one that is 20" wide (or 25% bigger), then you'd need to increase everything on the pack by 25% to make it the correct scale. Make sense?