how to transform 120V AC to 5V DC 5A load using flyback transformer circuit? Setting frequency - low cost, low footprint - change load capacity
how is the frequency set in a flyback converter circuit?


There is a chip (usually) that operates at a certain frequency, quite often 100 kHz but can be significantly lower or, up to a few hundred kHz higher.


  Which components are different when transforming a load of 400mA or
  transforming a load of 5A?


If the converter is designed to deliver up to 5 amps then there is no difference. If designed to only output 400 mA then, to use it for 5 amps will pretty much change every component.


  is this something i can find load measurements on my components for


You need to look at various flyback converter designs to get a feel for things.


  which components measurements should be reviewed when changing the
  load capacity to a greater amperage?


I wouldn't recommend this - trying to make a low power flyback converter suitable for a significantly higher power will require wholesale changes and is much more trouble than it's worth. You would redesign completely.


  So it does not matter whether it is AC or DC on its first input?


Usually it doesn't matter but, you do need to put a bridge rectifier (and capacitor) on the front end of a flyback DC to DC converter to make it suitable for AC applications but, there may be a lot of other details that need to change too in order to meet EMI specifications. A 30 volt rated DC to DC converter with a bridge rectifier and smoothing capacitor will not be able to work with AC voltages higher than about 20 V. It has to be designed properly as in all circuits.