HTML5 Canvas: Finding the height of a symbol

Hey guys

I'm probably missing something really obvious here.

I have an image within a MovieClip in the library and I'm adding it to the stage using this code:

var sym1 = new lib.symbolName();

this.addChild(sym1 );

However, I need to find the height of what I have just added to the stage, I have tried the obvious alert(sym1.height) but to no avail, I just keep getting 'undefined'.

How would I go about doing this?

Thanks