As we know, the regulation size 9X14X22 won't fit in the overhead on this soon-to-be-retired aircraft. But you've already done your homework, and your bag is smaller! I've employed a successful strategy for this on the E145. Get a green tag from the gate. Show it to the flight attendant and say that you know the bag will fit because you fly the E145 all the time (and, importantly) but if, for some reason it doesn't, you're already prepared to walk back out and gate check. Be VERY nice. It just may work.