The size of the plate can vary as long as it's above the minimum requirement. Some UK prestige makes have had a tradition of large number plate housings at rear, particularly Rover with 1960s 3-Litre and 2000+ Rover 75. Aston Martin are just ripping off the idea. However although plates can be bigger than the minimum, the letters and numbers have to stay constant size.

Some owners of eg US or classic Italian cars use smaller plates similar to original country format, but these are illegal on the road (but perfectly acceptable off road as show plates for display).