I just had a thought... If you can't figure out what size to buy, find the type of leg-band that you want to use, find out who makes it, call the manufacturer and/or the distributor and ask them for a couple of sample pieces in a few different sizes. We have do this all of the time for...
A size 18, sized in inches, would be 1 1/8", which I haven't seen.
Size 18mm = ~11/16", or size 11.
All I have seen so far are 4-16 sized in inches and 6-22 sized in mm (6 = 1/4" to ~11/16").
Does that make sense?
Here's Green-Pea (six month old hen) in a 10 and a 12. I know they aren't the numbered type, but it should give you a basic idea of how they might fit.
Size 10 - Too small.
Size 12 - A little too big.
This link shows info on numbered bands. I think the picture the you posted is for a size 9 which is 9/16".
http://www.strombergschickens.com/prod_detail_list/numbered-plastic-bands
My 6 peateens, ages 4-6 months, are all in a size 10 (5/8") right now and they're getting too tight, so I'm going to change them to 12's (3/4") today.
#4
1/4"
Small Baby Chicks, Adult Quail, Small Pigeon, Doves, Bobwhite, Coturnix Quail, small Pigeon, adult Bobwhite Quail
#5
5/16”...
These are sized in inches, size 4 (1/4") - 16 (1"), and I think that's the inner diameter.
http://www.cutlersupply.com/cart/index.php?main_page=product_info&cPath=6_88&products_id=563