in content_mc, i did TextField._height = output.lenght /2 which is 615; and i want to set the same height for the content_mc so i did _root.content_mc._height = _root.content_mc.TextField._height; it gives me the size 15.15; why?
The first thing you should do is not use "TextField" as a name for anything. It is an AS2 class name, so that might be causing you a problem.
What is TextField supposed to be? Why can't you just assign a variable to the content_mc and use that...
content_mc.heightVar = output.lenght/2;
and later use
content_mc_height = content_mc.heightVar;
because you asigned content_mc's _height before the textfield's height changed. the textfield's height wasn't changed, when you tried to assign content_mc's _height, either because you didn't set it correctly or you're using a component where there's a delay in the textfield size change.