Standard size of rear number plate

Hi,

Does anybody know what the standard size of the UK rear number plate is on a 92 (or similar) NSX? As I think is common mine is larger than the average rear UK plate (in height rather than width), and as I plan to replace soon I want to get it right. I can measure and report back if this makes no sense.

I mentioned to Chiswick Honda today as I think I'll replace when it's next in for service, and they gave me spiel about there only being 1 legal size for UK cars. Is this wrong?

Cheers,
James