Flat couplers are just two pieces of leather of whatever length you want and can even be different colors so you can walk two dogs comfortably. 

Width of Leather
Length - each side, leather only
Color 1 or side 1

On the Hardware, the Large and XLarge are for the XW leather.  The Standard is for the 1/2", the medium works well on the 3/8" and the small is great for the 1/4"

You can choose other combinations if you want but remember the base of the snaps are sized for those leathers.