What is the maximum power that I can get from a flyback controller?
There is no inherent limit to the power you can get from a flyback topology. It just happens to be more cost-effective for relatively low power applications. Many flyback converters control the peak current by a low-value external sense resistor in the switch source (or emitter) circuit. 

I suggest auditing the Coursera courses on switchmode design (from the University of Colorado, I think). They're free to view (but unfortunately not the quizzes).