Just measured them filled with water to the brim (note, not working capacity, but total capacity)...
10cm = 750 mls (500 mls realistically = 1.25 tins of beans)
Hardcore this one. Really too small for much more than a brewup - same total capacity as a crusader cup. OK for a daypack, the genuinely hardcore bushcrafter or the space concious backpacker, but for most I would think it's fiddly small for anything other than boil in the bag. The inner tray isnt really worth having. This billy makes a good companion to British Army mess tins.
12cm = 1250 mls (1000mls realistically = 2.5 tins of beans)
Best size for backpacking IMO. One luxury sized toilet roll fits perfectly inside without the inner tray. Fits well on westwinds, mini trangia and Clikstand stoves. Smallest size you can actually "cook with" IMO. The inner tray, though small is functional as a bowl or small frying pan for maybe 2 sausages. Fine on it's own, but luxury cooking with mess tins.
14cm = ?
Dont know, I havent got one.
16cm = 2750 mls (2400mls realistically = 6 tins of beans)
McMears family bucket - at almost 3 litres, almost the whole capacity of a large camelback - too big for backpacks, but great for car camping. The inner tray is huge, full roast dinner on this no worries. A good functional steaming tray, or use as a frypan for your full english. You could cook a chillie for 6 people in the main billy.