QFT. As a versatile guy, I have have 8", 7", 6" and 5.5" back there with girths ranging from 4.5 to 6." They have all been very pleasurable. As I have said before, I have had my brains fucked out of my head by a guy with 5.5" who knew what he was doing.
The stabbing effect is more a result of the top not knowing how to bring pleasure to the guy on the bottom. Guys with less length have to be as mindful of hitting the sides the wrong way as guys with lots of length have to be concerned about hitting the second hole before a guy is ready. I did that once and he let me know really quickly.
Size is irrelevant to me when I bottom. I find chemistry far more critical. As long as a man is long enough to get past my (rather ample) ass and knows what you are doing, we'll be fine. If not, well, when it is my turn to top..payback is a bitch! :wink: