My ex-boyfriend's penis was 10" x6", and I could never manage to deep-throat him. However, during a threesome, the other girl did, and it was super easy for her. Because of that, I knew it was possible, and never succeeded, but also never gave up. Next really big dick I came across was 8" x 6", and I could deep-throat that one all day long. I think part of it was continued practice, part pent up resentment of my ex, and part head shape. If the head is flatter on one side, it does penetrate the throat more easily.