I design a strap firstly around the required shoulder-pad width to suit the instrument.
If you have to think about the weight on your shoulder you cannot concentrate on playing.
The widths - and their matching tapers - given on the Options page are based upon years of informal trials - and on aesthetics, of course.
I use heavyweight leather, about ⅛" thick or more, which is durable and spreads the load.
3⅜" doesn't overhang most people's shoulders yet is wide enough for fairly heavy basses, whilst at less than 2½" typical electric guitars might start feeling a bit weighty, although that does depend on the design - especially the wood used.
The sizes shown on the Option Details will cover most players' needs, but a little wider might suit some people and a mandolin player might prefer something... rather narrower.
Please ask if you need an alternative size.
Shoulder-Pad Width - The Key to Comfort