My limited experience is size doesn't matter. I've been fucked by a guy with avg maybe 5.5 and it was uncomfortable. He went too fast. I was fucked by a BBC (8 in and thick) and it felt awesome. Couldn't walk when he was done but awesome. We took our time to get it in.
To answer your question. I prefer thin 7-8 inches.
For me, not interested in being topped, just stroking another male cock. Size sometimes does matter. At least 6-7 inches of the best for mutual stroking and trying out frottage! But, the best is guys with fully cut Cocks! Paul
The longest I ever had was a beautiful Jamaican guy who had 9.5 inches of thick black meat, but the best was a guy who had 4.5 inches - God, he fucked me like a steam-hammer!