There is no separate "Sheet size for a movie clip". For any flash file the stage is the only main reference point where objects can be placed and moved around.
But in your case you can achieve what you wnat programmatically using code.
Instead of loading all the information in a single movie clip, you can break it down into smaller movie clips and then load the corresponding movie clips as you scroll through the content.
Hope it helped
well, flash has a limitation of 2880px what you could do is when building your movieclip is shift as far as it goes to the left and continue adding objects (in your case I assume textfields), couldn' recognize it on the screenshot. and then move everything back to the original position.
Best way though would be to create a small function that adds the objects and positions them at the right place (in AS there is no size limitation.