http://www.woolworks.org/patterns/raglan.html
Here is a customizable pattern you can try to make any size in any gauge. I've never tried it myself to vouch for it, but it sounds like a great idea.
http://spincraftpatterns.com/29swSuretoFit.pdf
And here is another...scroll down to about page 5.
I see they both use inches and are not metric, hope that doesn't make them too confusing!