We need to post a picture of ours. The standard plate looks just fine plus it is legal! I could not summon the courage to do the job so took the car to my local guy.
Ive had a 3/4 size one on my Vantage for 5 years ...dunno the measurements, just ordered "3/4 size" from fancyplates or GTgrafix online (cant remember which)
lots of pics on here on plate threads of various sizes