Set symbol's visible area - I can't 'get' it

I've been using Edge Animate for a short time, but still can't figure out how to set the "visible" area of a symbol. Here is what I am trying to do - Create a box on the stage that displays pictures and text that can be scrolled with a scroll bar.

I create elements on the stage and convert to a symbol. It seems that I should be able to now edit that symbol and reduce the "height" amount to cause scroll bars to appear. But, this has no effect on the symbol. I placed the symbol on the stage and set the display to "scroll" or "auto" and it does not care about the new height setting.  What am I missing?