This was my old plate.
I then had it changed to this.
I got pulled over one night with the smaller plate, it had motorcycle letters on it, and they were only spaced 5mm, I was going to get fined but seeing as it was the first day I had driven the car they let it slide and gave me 30 days to change it. I got my new plate made following the guidelines on the dvla site. I got pulled over one day because of the size of it, however, I now carry the printout from the DVLA in my car, the same one that mojor sarcasm put up a link for. I showed it to the police, they measured up the plate and they said it was fine. The letters are still motorcycle size, but they are now spaced 10mm apart, although the plate is slightly larger, it was not the height, it was the width, it hardly makes a difference as you can see from the pics. Hope that helps.
Here's the dvla website link again just incase -http://www.dvla.gov.uk/media/pdf/leaflets/displayofnumberplates.pdf