Hi,
The front number plate should really be the standard UK size as specified by the DVLA.
This web site should help.
CLICKY, although it only covers the letters and their spacing (mianly for automatic number plate recognition devices, like some speed cameras and the London congestion zone cameras)
The front one is best being all on one line like this (IMG:
http://mysite.wanadoo-members.co.uk/adepics/images/3-picture2.jpg)
There are some around like the rear one (square with the text on two lines) but I think the legality of these is a bit grey, some people get away with them some are told to change them (by the police).
We tend to get away with the rear one being square because the recess in the rear bumper is square and there isn't a UK equivilent bumper to swap it for.
Hope this helps.
Ade