I think mine's 6x4" and I've had no trouble with it. I was told when I bought it that it should be OK-any smaller would risk a tug. You can get an idea of the size by looking at my photos (sidevu and midbelow).
A hog-riding mate of mine has one that's even smaller (and silver-on-black) and he's had no problems, either.
I think the cops are on the lookout for sportsbikes with those tiny plates, loud cans, etc.
Just buy it, fit it and plead ignorance if caught!!!
