The CRJ700 and 900s that I have flown do not always have moveable armrests in between the seats for the bulkhead row, and would be uncomfortable if you are of size. I prefer 5C on the 900 as I am only slightly larger, fit fine in between the immoveable armrests, love the extra legroom, and it keeps me from overflowing into my neighbor's seat.
I'd move back a row to 6 if it's available. Sometimes the aisle armrest will also go up in rows 6/7 - look for a small button on the bottom and all the way at the back of the armrest.
Also, as another poster mentioned, the bulkhead aisle seat that is 4B or 5B on those planes gets bumped a LOT due the position of the seat to the aisle itself.