I don't think I would try to shrink them, you don't know how they'll turn out and then you'll be stuck with expensive but useless arm warmers.
I have an old pair of Bleu Forêt silk/wool knee-high socks (with very cool stripes) that is quite worn at the heel, with assorted holes in the toe area. I'm going to cut it and somehow finish it (I'm still trying to figure out how) and wear them as arm warmers. They fit just fine and hold on to my stick arms, unlike ALL other arm warmers out there.