An erect penis that is so large it's beyond comprehension or appreciation; or one that inspires fear of pain during an anticipated sexual encounter.
The determination as to what constitutes "scary big" is somewhat subjective.
-- "How'd it go with John last night?"
-- "It was a no-go. I got a look at it -- it was scary big! I couldn't imagine doing anything sexual with it or to it, and I had to beg off."
-- "Well, how long was it, anyway?"
-- "I don't estimate inches, but at least eight."
-- "And that's enough to scare you? Girl, you ain't lived."