Not sure there's a limit on size, but the characters have to be set out in 2 lines, and have size and font specifications. There are rules about spaces between the letters and the edge of the plate etc.Thanks !
Here,in Poland mounting licence plate is legal as long as you don't drill inside letters and numbers .I've drilled mine on the EU stars singn and hologram as well
How about plate dimensions in the UK?They looks slighly bigger then ours here.(about 190mm X 150mm in vertical)
We can drill through the letters/numbers, you just use an appropriate coloured bolt (they're plastic), so if you bolt through a letter, you use a black one, if the plate, a yellow one. (or white for front of cars).
It's not uncommon for bikes to have really small plates. They're probably illegal, but they have them. Mine is standard, and probably about the same size as yours, give or take a cm or two.
I think your tail tidy would fit OK, maybe the bottom would need to be shorter if someone was using a small plate though. I'll measure mine later and tell you the size of a standard one.
Are you planning to sell them? I was thinking of making one myself, but yours seems to be better than anything I could do.