My daugter-in-law measures 15" for loom length.This seems really short. At what point does boat width become a factor?
Matter of opinion
I don’t go along with the boat size thing (I think paddles should fit paddlers) - but take that as coming from someone who’s boats are all narrower than his shoulders.
Some folks bring hands in or out when doing that measure. Sometimes this puts them in best place, sometimes not. Assume some error. Check shoulder width too.
Only thing I can really add, is to consider the paddler’s age/fitness/style. More active types may like/benefit from a little longer loom (5-15%). I would consider including just about anyone under 30 in this automatically unless really unfit.
Side note about sizing to boat width: GPs were designed to work with narrow craft craft and paddlers low to the water. This isn’t goed marketing, but IMO trying to adjust them longer to suit wider and or higher kayaks may work, but won’t allow the GP to be much of a GP. Wider than 23-24" of any type, probably better off using a Euro. Higher seat narrow kayaks with larger/open cockpits like SKI/K1 - best with wing. There are also other traditional types designed to go with wider/deeper kayaks that are worth looking at. Interestingly, some look ore like euros.
15 " is narrow
but I made a GP for a friend with a 15" loom and she likes it. She is an adult woman who is very petite with narrow shoulders and after trying out paddles and other sizing experiments she insisted on 15 inches.
My personal paddles have 18" looms which is also on the narrow side but she insisted that here hands were spread apart to much so I went with the 15.
If that is too narrow the loom can be increased easily.
I’d say try it.