The second one is correct if you add "that" to it: "It's not that big a deal" There's actually an old article on the Examiner website about it. The phrase that is incorrect it "not that big OF a deal" http://www.examiner.com/article/how-big-a-deal-is-that-big-of-a-deal In terms of use, I'm more likely to say "it's not a big deal" unless I really wanted to emphasize the point, when I'd say "it's not THAT big a deal"

Hey there! "It's not a big deal" is certainly the right way to say it, and you were taught correctly, what you might be hearing on TV is "It is not that big a deal" or "It is not as big a deal". "It is not big a deal" is not grammatically correct and sounds awkward.

"it's not a big deal " is the correct one grammatically , it's what i always hear in formal english , however , " it's not big a deal " it could be correct but as an informal expression .. since english is not my native language i always use the familiar expressions .. it's the best to learn

I would always say "it's not a big deal." However I am British. I think it's possible to also say "it's not that much big of a deal" too!