Even small bolt cutters will fit 12mm links in their jaws. I think you want a hardened chain, so at least they struggle to cut it.willevans said:
I've seen plenty of videos of 48"ish bolt cutters going through even the largest chains. At least one of those videos was of a thick ABUS chain (and highly regarded), which should have been hardened.
I have two chains, an 8' ABUS noose-style in 12mm square links, and a 5' Kryptonite with 11mm hex links. The noose-style is more useful, but the Kryptonite is way way burlier.
Check out http://www.lockitt.com/chain.htm for a variety of chains (and if you follow the links up, cables), mostly intended for motorcycle use, but also work well for locking bicycles if you can leave it in your trunk when not in use.