"I do dog sitting and dog daycare and I have these in three sizes.
The sizes can be a little hard to determine (and seem to differ from what is printed on the packaging) so below are the approximate/hopefully more accurate measurements.
They do shrink a little when washing, and the velcro is 2" wide to allow for some adjustments. You do want these to fit rather snug so they don't 'roll' on the dog or slide off when they play/run.
SMALL: (perfect for Yorkies)
11.5" (without elastic fully stretched)
17" (with elastic stretched)
This is a new diaper.
MEDIUM: (Great for West Highland Terriers, Welsh Terriers, Bassett Hounds, etc.)
17" (without elastic fully stretched)
25" with elastic stretched)
This is a new diaper.
LARGE:
20" (without elastic fully stretched)
32" (with elastic fullyl stretched)
This size is for a diaper washed once in warm water and machine dried."