Get rid of the superfluous of. The correct expression is not that big a deal, not not that big of a deal. The of serves no grammatical function and should be avoided. Why does it nonetheless intrude so often in that expression and similar ones (not that tall of a guy, not that good of an idea, not that fast of a runner)? I have no idea. What I do know is that it coarsens the language and should be exterminated.