How do I find the height of a control that lacks a 'style' attribute?

I have a control in my page that I need to know the height of. How do I find out the height, if it lacks a css style attribute for height?
The reason I need to do this is that the control is an asp.net control, its a "textbox". It does not have a style property, as far as I can see.
After I obtain the height, I want to change it programmatically, which I know how to do (just set the style.height property)
Thanks