Hello Dave,
This may come a little late, though I hope not. Following Paul´s advice about looking at Fairmile Ds, their hull numbers were 375mm high and 300mm wide, according to John Lambert´s book.
I suppose it would be quite hard to paint them individually, so you may want to try Becc Flags. They make self adhesive lettering (they have Royal Navy pennant numbers) to different sizes. Just google them and you´ll get the link.
Cheers!