I know this is pretty late and maybe you've found the solution by now but for others who have just started Flash or have stumbled upon this question because you were stuck (just like me) this is what I discovered...
So I had added my content already and I wanted to resize my stage without having to resize my content or do special 'stuff' with it. How I solved this:
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1.) I made sure everything was locked first, then I right clicked the stage.
2. ) Clicked "Document" (or Crtl+J)
3.) Then after that you get a pop up with one of the options... "stage size" "_____x_____" <--enter your desired stage size
[I changed the units to inches to make things easier and then I played around by entering different numbers into the boxes until my stage was at the perfect size I needed]
4.) Click Ok
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Content stays the same. Stage size changes.
Hope this helps! : )