Personally I'd stay with a full size plate, simply because I think the ultra small plates that wrap around the length of the characters looks stupid in my opinion - far too boy racer for my liking. Also I'm not too sure whether its legal, have you checked this?
My neighbour has a three letter registration on a full size plate and it looks great!
In regards to what will fit, it depends on if your original plates have been drilled and where in fact they have been drilled, relative to the size of a standard plate. If they have been put on with adhesive then you could go as small as you like.
In my opinion - I think you should keep the full size plate, if its good enough for Lord Alan Sugar on a Rolls then...
Yep it's all about the spacing and letter size, I haven't had any issues with my smaller than standard plate in the last 10 years, I got stopped before that for the wrong font/spacing, oops!
BB is right though on some car plate mountings the smaller plate can look lost so you might want to try a cardboard plate mock-up first and squint a bit to see if it looks right.
I haven't got my car yet but I noticed the holes in the boot are quite wide apart, does anyone know if they are in a fixed position as I've got my plates pre-drilled already?