The legality is that that there is no law on the size of a number plate, the only law is on the size/spacing of the lettering involved, these are:
Character Height: 79mm
Character Width (except the figure 1 or I): 50mm
Character Stroke: 14mm (the width of the letter I and the width of the lines used to make up other letters)
Space between characters: 11mm
Space between groups: 33mm (ie. between A123 and ABC)
Top, bottom and side margins (minimum): 11mm
Space between vertical lines: 19mm (ie. on lorry plates)
This tends to lend itself to the two standard sized plates we see on the road today, however if you have a UK car and own the plate "A1" you could legally have a plate 119mm wide.
However if you car is an import and "its design cannot accomodate a standard size plate" then the lowest you can go is:
Character Height: 64mm
Character Width (except the figure 1 or I): 44mm
Character Stroke: 10mm
Space between characters: 10mm
You'll notice there's no requirement for a border or a space between groups.
Basically you can fit a motorbike plate, however in practice people tend to fit the plates sized after the original country's plates (approx 13"x6.5" for Japan) even though this is wider than it needs to be. The other common one is if you own a JDM car where the number plate hangs off the bottom edge of the bumper (Such as on the Skyline) you register a 5 or 6 digit private plate and go for the JDM width but UK height so it doesn't overhang any more. This is what I intend to do on my Cefiro.
All other number plate rules such as no graphics apart from the Euro/UK/Eng/Sco/Wales/NI flag and the requirement for the plate supplier's details still apply.