I have a 7x5 carbon plate on my 749. That is the width of the bracket I got. http://www.casoli.co.uk/products/casolijh.htm
We were stopped in a Sunday "safety check" by Thames Valley police. My mate was nicked for a 2inch hight plate and another for race cans. They looked at my plate and let it go.
There is a specification for plates that includes not only character hight, but also the font used, the sapcing, the yelloq border width and (beleive it or not) it is also a legal requirement to have the dealer name across the bottom of it.
Obviosuly it depens on whether they are keen enough though (revenue collection might be down that day!)
I don't use that plate now as it broke at the mounting holes - took someone on the pillion, and going over a bumby bit of road the suspension grounded catching the plate on the tyre which dragged it round and also the exhaust mountings damaged the carbon hugger. Dealer said to increase compression damping half turn and that felt better the next time!