I don't know about the size, cops can have ago bit i know 100% it's a legal plate, if you have the post code of the plate producer and the british standard mark and the letters are the right space apart and all that rubbish then its all oki and legal.
I went to halfords and said i want a square plate, maybe the two sizes is the motorcycle square plate and the car square plate. I haven't attached them at the top i have attached them one on the left and one on the right set vertically in the middle. Its holding absolutley fine so you only need a couple of screws.