In answer to my own question, Transport Canada recognizes three levels of passenger air transportation: air taxi, commuter, and airline.
An operation is commuter if the airplane has more than one engine, weighs no more than 19,000 lbs and has between 10 and 19 passenger seats, or if has any number of turbine engines, up to 19 passengers seats, and weighs no more than 50,000 lbs before you put any fuel in, or if Transport Canada has authorized it to be commuter. Smaller than that, an operation is air taxi and bigger than that it's airline.
I think passengers stop shrieking in horror about how small the airplane is at around 100 seats, B737-sized, but I do remember hearing someone who had only ever flown transatlantic express surprise at how small WestJet airplanes (B737) are. And I remember a student pilot gasping in astonishment at the shear size of a a Dash-8. It's all what you're used to.