need help with sizes.

hi.
me and my girlfriend are new to this but we are really into wearables, but it seems i can't get the sizes right, i ordered David's sheath medium size it was taking so long i bought crackers sheath small ready made small one and it came first it fitted on really good but didn't covered me all the way as i wanted we really enjoyed it and couldn't wait for David, it came and i couldn't fit it on it was to small for me. so now I'm confused can anyone help? have pictures with cracker but non with David because it didn't fit.

 

Usually I just go by the internal measurements and match that to my fiance. They're not all gonna be the exact same lengths.