I'm lumping the 175 and the 900s into the same category, because they are the newest Rjs in the NWA fleet and therefore will be treated as such. They'll be flying on routes that were formerly flown by avros and DC9s and even CRJ200s. The bins are indeed bigger on the 900s and it isn't just perception.
AZJ
Just because they are the "newest" does not mean that they will load carryon luggage the same and/or closely resemble one another or their predecessor aircraft.
What you get on a CRJ 900 is a first-class seat (grab some padding for your back) plus a drink & snack en route.
Here is Bombardier's illustration of how "big" the overhead bins are:
They are the
same size as the bins in the 700's. I've flown on both 100/200's (NW, DL, UA) and 700's (DL) dozens of times each (as well as over a dozen trips on NW & DL 900's), and believe me there is
no appreciable difference in the sizes of the overheads. Full size rollaboards (and generally anything with wheels) are gate-checked on all of the above.
Bombardier's claim they they accept a full-size rollaboard is a joke, especially given the one-plus rule. When (if) they fit, they go in sideways only.