There's no such thing as a penis being too big or too heavy. The vagina will stretch to fit any size penis and there are plenty of men with above average size penises who can get and stay hard with no problem.

I believe your problems are all inside your head. You are making way too big of a deal out of it and you're mentality is not allowing your penis to get hard enough. You are too awkward and uncomfortable with yourself=no confidence enough to stay aroused.