Frosted (fronsted??)insides ones it should be then...
Nowt but the best I would suggest.
In terms of the graphical appearance, I'd suggest that Dave would best placed to suggest how that would best look but I don't want to keep putting on you Dave...!
Sizes, who knows. I would have assumed that it would need to be large enough to be ledgible to other people on the road, but agree that it shouldn't be too big otherwise we'd be bordering on Max Power style decals...
I dunno, what's the standard size of the decal that your local friendly garage put in the rear window? That size ought to do it...