I once did some maths and some research and came out with something like three earth radii, if you push the density and rotation speed as low as you can. I can't remember how i came to that particular number, so don't believe me.
Of course, there are other considerations that worry me about three-earth-radii worlds: for example, wouldn't such a large world be a lot more stratified than our own, thus having a much lower concentration of iron and other heavy stuff on the surface? not enough of a low concentration for it to have biological effects, like no one's getting anemia unless they don't eat properly, but we're still considering civilization-spanning differences when we start considering mining and whatnot. I mean does a world without metal technology look like? We've got *some* inkling of that amongst island peoples, like The Conworlder 1 played with the idea briefly but got distracted from by an episode of Doctor Who.