The Clover Natural Fit Leather Thimble is made with a double layer of leather throughout, sewn 3-dimensionally to fit the shape of your finger, and has no seams or stitches in the areas where the needle touches, meaning you'll never stab or poke yourself. 
 
Personally, it's the only thimble that has ever worked comfortably for me. I wouldn't do any kind of hand sewing without it.
 
Medium - Inner diameter approx 16mm (5/8 inch)
Large - Inner diameter approx 17.5mm (11/16 inch)