Using 4mm soldered round, 2.6mm inside diameter, 22 gauge rings, I can loop connect with 4-6x tippet material. My typical loops are tied with double surgeons knot. I suspect tippet size is governed by I.D. of tippet ring.
BTW I recently have noticed that whenever I must break free my snagged tippet, the break is usually within 2-3" of the connecting loop. Tippet material is mono and not fluorocarbon. Anyone else notice this?