The blast radius is small, but don't let that lull you into thinking you're fine if you're not near it; a major component of the damage done by a nuke would be the fact that it ignites all flammable (and much considered not flammable at relatively low temperatures) materials. Even a 5kt nominal yield weapon going off in lower Manhattan would set fires as far away as Brooklyn, Queens and Jersey City.

Reference: http://www.thebulletin.org/article.php?art_ofn=jf04eden