I have a small mag/fero on my keychain which is probably no bigger than a AA battery. It is predrilled with a hole for a ring, and has a fero rod glued into a channel on a round bar of magnesium:



I got this a long time ago, it throws some great sparks. Although there's less magnesium than a full bar, I am amazed at how little magnesium you need to get your tinder going if it's well prepared. I'll try to find the place I bought this and post it later.