My personal item these days is the High Sierra Swerve backpack. I travel with 2 laptops and has space and holders for both; so a large pack is a must. Published dimensions: H 19" x W 8.5" x L 13". 4 compartments, plus side pockets good for a waterbottle and a paperback. It does fit under the seat in both mainline and RJ/Rprops.
BTW My Regional carry on is a Red Oxx Aviator small, which fits in all RJ/Rprop overhead bins (other than the left side of F in CRJs that have F).
Looks like this:
