My front plate is 305mm by 76mm. Been stopped twice now and fined once, as a result of this my big gay normal plate has had to go back on for a while. Think its because the lettering is pretty small, it passed MOT but then again all my previous cars have because i've known the garage owner for a while haha. Think it just depends on what copper pulls you over, the one who gave me the fine was on a bike so turned round pretty quickly and then decided to inspect everything on my car like tread depth etc, whereas the second time i got pulled they just asked me to put the legal one back on (which i had i the car ready to stick on quickly!) and then had a chat to me for ten minutes about s15s!