They are all designed to fit the standard MGM size for Blu-ray covers (here in the U.S.) i.e. 3165 pixels by 1748 pixels. (in inches that translates to 10.55 inches by 5.827 inches / In mm that translates to 267.97mm by 148mm).

Blu-ray covers in the UK tend to have a slightly wider spine on them (at least the region free BDs I have ordered from the UK do).