I have had the hardest time finding a smallish metric proportion wheel. I have looked in the art stores and on the net to no avail. The closest I can come is a medium sized one 6" that is in both inches and centimeters.
Ideally I would like a 10cm (4") proportion wheel with metric measurements.
I could make one myself I guess, I do not think it would be that hard.
Edit: I ended up buying the large proportion wheel that is metric on one side and imperial on the other. I realised as I'm not carrying it around on me it does not matter if it is big.
If you can not find a proportion wheel for your art then maybe consider this. Most comic artists will work at %150 bigger than the finished product. This helps that art look neater and tighter and all round nicer. All you need to do is know your desired size, ie, what it will be printed at then just times that by 1.5 and you'll will have the size you need to draw it at.
Desired/print size = 13.5 cm
Times this by 1.5
Equals the original art size = 20.25 cm
Whilst searching for one I came across these human body proportion wheels for making dolls...