Although I just read..
- The margins at the top, bottom and side of the plate
must be 11mm.
So yeah you could make it shorter in length and should still be legal but the amount of coppers who don't know the law and might pull you anyway, it's not worth the hassle imo. I have 5 letters/digits in my plate and stick with the standard sized plates because I don't want the hassle.
And as said the best place to put it is where it was from the factory. People put them at the bottom near the splitter but it makes your car look like a goofy kid at school imo