The Saab 340F is one of the smallest commercial planes I have flown on (but not the smallest). It has three seats across, one on the right of the isle when facing the back of the plane, and two on the left. While it is small, there is some small overhead space, and they can run drink services (if the flight is long enough) and there is a flight attendent.
This is a picture of a Saab from a Northwest flight I took to Champaign, Illinois. You can see that the plane is a twin turboprop. This helps enhance the vibration and noise inside the aircraft :-). Like most commuter flights, you walk out to the plane and up the fold out steps.
Watch your head! Low clearance!
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