A regular size rollaboard carry-on bag does fit in the overhead of the newer CRJ-900 / CR9 (as compared to the CRJ-200 / CR2 and E145, where it definitely does not fit). My luggage is 21 x 14 x 9 inches and it does fit in the CR9 overheads, although it is a tight squeeze, there is no room to spare. Two bags of the specified dimension fit lengthwise per bin.

On Delta the GA will let you board the CR9 with this size bag, especially if seated in F (they tend to gate check later in the boarding process and obviously oversize bags). But on AA, they tend to give a gate tag to everyone and insist that it will not fit, presumably because they don't want to be bothered to use the sizer and a lot of bags that people bring are too big. I have seen limited success on AA with people taking the gate tag and then ignoring it and boarding the plane anyway. Your odds of this are better if seated in F as you board first and put it up right away and there is no question it fits. But you will occasionally be told by a FA at the door to the plane that no roller bags are allowed on the plane at all. In that case, what others have said about gate checking vs valet checking is true-- you will be given the chance to valet check your bag, and it is not worth arguing. Thus the only question is whether you will be able to bring the bag on the plane or retrieve it in the jet bridge, you will not have it sent to the carousel.

To be clear, what I am saying only applies to truly "legit" carry-on bags that meet the dimensions and fit comfortably in the sizer, not some of the enormous/expandable bags with crap sticking out in every direction, that "fit" in a mainline plane.