I have never swallowed a really long cock, but I would think once you get the head of the cock past the entrance to your esophagus, it's all the same from there. More experienced swallowers please correct me if I'm wrong.
As to thickness, I have swallowed my husband's 6 inch thick cock many times, but this is a very, very tight fit and is painful for both of us. My mouth is only so big, and I simply cannot keep my back molars from pressing into his cock in that situation.
I did not have the same issue with my ex husband's 5 inch girth cock.
My current husband,
ArtfulDominant, never thought
anyone would swallow him, though, so he loves to have me do it, even though it hurts.
I can't imagine swallowing an even bigger girth -- and I don't think it would be much fun for the guy, either.
I think I could swallow a longer length, though.