Especially for those not blessed with ‘Hendrix hands,’ is there a rule-of-thumb for estimating the correct guitar-neck for a player’s hand before actually holding the instrument?

I’d say no – we play with any hands/fingers both smaller (uke/mandolin) and bigger instruments (cello/double bass). My standard guitars measure between 23.5 and 25.5 inches in length and nut widths from 1 9/16 inches in Gison’s ‘pencil’ nut width to the 52mm (2.05) fretboard of a classic guitar. The Mustang (2 “”) is my current favorite. The first four frets should be covered comfortably.