I think, technically, that you are wrong about the size of the front plate David. I have a personal registration with only 5 characters and have taken the opportunity to have a shorter plate at the front - the type, size and spacing of the characters is to the letter of the law, as is the height of the plate, but the plate is slightly shorter, and fits perfectly on the raised area on a pre-facelift car, which is rather neat. I think the arrangement I have is technically illegal but since the appearance and size of the lettering is 'normal' I have yet to find a copper sad/churlish enough to pull me up for it.
Regardless of the number of characters though, I think I'm right in saying that both the front and rear plates should be of 'standard' length, even if it would mean a lot of empty space.
A small point to note though, the front bumper on a facelift 156 sadly has no raised area at all for some reason, as I found when I went from my 2002 Selespeed to my current 2005 Sport. I intend eventually to fabricate a plinth to simulate this however.