Thanks for the update, I have just called the UK Information Commissioners Office (ICO) and they confirmed that there is no Legal minimum size, the only requirement is that it must be visible to the public.
As my property is a private residence I want to stick to the letter of the law and have put up signs adjacent to my cameras. I've been advised by the Police that they have no objections to CCTV however, I want to cover myself legally due to a vindicate person who lives a couple of streets away.
As you can appreciate I dont want to put up great big signs as A: They detract from my property visage, and B: they attract attention that there is a camera there and potentially makes my house a target.