There is no minimum or maximum 'legal' size of numberplate. It's the character height, width (whole character and width of font stroke), font and the spaces between the characters and edges of plate that need to meet a legal requirement.
I've actually been stopped for my previous plate and was made to change it, despite it meeting all DVLA requirements.
Bottom line is, as the front bumper of the GTiR, can accomadate a usual EU style long plate, that's what you need to fit legally.
Forget what plod at the side of the road tells you, as they generally know nothing about these specific areas. I actually spoke to the vehicle inspector unit and they were the ones who advised me what I had to have fitted legally.
However, if you drive the car sensibly, you're unlikely to get a tug. With that in mind, I'd put on the front of the car what you like. It's only about £10 to get a legal sized plate made up, if you ever need too ! ;-)