It's all about the getting the look right and having everything in proportion rather than having a feckin great airbrake on the back!
taxdiscolish 1 on ebay ,they have a white backing though if that bothers you . i just sent a message to him stating that it would be a legal plate size if it had the number 1 or 11 in it . i have had to cut a bit off each side of the 9x6.5 plate as they don't do an 8x6.5 , but the 7x6.5 came the right size without the need to cut it.
I had a 7 x 5 but it looked too small for my taste. Replaced it with an 8 x 6 and it looks right. I got pulled over by the boys in blue last year and they never said anything about it.