The stator is just the part inside the motor with the metal laminations, where the windings are. It's probably only 28mm, if it's a common 9C/clone type. That's the reason for the common designation 2807 for a 7-turn, or 2810 for a 10-turn, etc.
The magnets may be wider or narrower than that; whatever the narrowest of the two (stator or magnets) is the effective "width" of the motor, as far as torque production goes (or passing heat using Statorade).
Most of these motors are easily capable of 1000-1500w, as long as they're doing that at a fast enough RPM to be in their efficient zone for the voltage they're being run at. If bogged down at those power levels, they'll overheat and burn up eventually. (see http://ebikes.ca/simulator
You can probably even run bursts of 2000w+, but it might not run all day that way.
Higher power...depends. At some point, you run into the point at which more current thru the windings just creates heat (saturation). It's different for each motor design and winding count.