On the 120, they're almost non-existent. You've got more space under your seat than you do over your head (and there's only one set of bins, over the BC seats; A seats have none). I have problems fitting a small duffel bag or my laptop bag (a Trager shoulder "messenger"-style bag) in those bins. If you mean the 170, it's got bins which are fairly large for a RJ, and larger than some mainline planes (such as the center bins on the 767).

The CR7 bins are larger than the 120's, but they're still RJ bins. IIRC, my smallest rollaboard will fit, sideways, into the CR7's bins, but I gate check it anyway to save the space. That's a tiny rollaboard, too; my more standard sized one won't even come close.