I just fitted one thats 330mm wide - it overlaps the hole just enough to cover it.
Unfortunately I only got it 170mm high, so I've got a gap at the top...
Luckily it doesn't show unless you're looking very hard.
So, the full answer is that a plate 330mm x 175 should do the trick nicely
Fancy plate(s) : Back plate only
Registration : R651 xxx
Font : Standard UK, Shadow : normal
Border : Border 3, Black
Bottom text : http://www.mkivsupra.net
Logo : Miscellaneous, No logo
Back : 330mm x 170mm
Honeycombe : no
Both white : no
Slant : no
Comments : Fit reg'n on 2 lines please
Price : £13.50