Measuring from centre of index finger tip to centre of pinky finger tip, in playing position (i.e. with fingernails facing upwards / towards you), how far can you comfortably stretch?
(BTW - I said left hand - I mean 'fretting hand', sorry to all you sinisters!)
I can (only?) manage about 5.25 inches / 13.3 cm.
Also (and this is why this is in the 'basses' section, rather than the anatomy section) - what scale length bass do you play? Wondering if there is much correlation at all. I am happy on a 34, but can do more on a short scale.
