I use 16mm on my Warren, ISA Brown and Marans (except the cockerels whose legs are too big) if that helps.
I got mine form Mark at Cockerdoodlequack (Ebay) and I'm sure if you ask him nicely he'll send you what you need. If you have any problems I have spares in red, yellow, blue and green. If you only need 4 I'd be happy to post you some. PM me your address if you have no luck with Mark.