Whitewoods - 3/8ths to 1/2
Dense hardwoods - 1/4 to 3/8ths
A nock generally won't fail unless you violate the back ring and even then it's still possible to get away with it. Remember that the stress in a bow is at it's highest next to the handle getting less out to the nocks. The tips of a bow have fairly small forces acting on them so you can go smaller than you think.
A bit of a misconseption IMO is that smaller/narrow tips reduce handshock. Handshock is almost entirely linked to the tiller of the bow, of course i'm talking about 'normal' tips here not some monster 2 foot wide lumps on the end of the bow.....