James Deen -- Hey, Farrah Abraham ... I'm at Least Medium-Sized!

Porn star James Deen has a HUMONGOUS sense of humor ... cracking jokes to TMZ over sex tape co-star Farrah Abraham's diss of his manhood.

An intrepid TMZ producer texted Deen to see what he thought of Abraham telling our photog she thought Deen "had a small penis."

Deen's response is the best -- "What is her problem???? It is obviously medium sized :)"

He added ... "On the list of things I care about the size of my penis is number 783."

Maybe the problem isn't that Deen has a small penis. Perhaps ...