For me, Vixskin Outlaw was the absolute limit, only possible due to the squishiness of the toy, and that was really pushing it. Even Vixskin Lonestar was alot for me.
So now, I stick to toys that are under 5.75" around.
My girlfriend went as large as the Basix Fat Boy (at my request), which is roughly 7" around at the widest, and decided that was just too big to enjoy. She prefers to stay in the 5.5"-6" circumference range as well. The biggest toy we still own is Adam's Pleasureskin cock, which is 6" around, and she only likes that large occasionally.