Where does the content height get set?

I've been working with a simple application using a treeview controls with multi-checkboxes.  The height is 315px which only allows about 10 lines of text. I can't find where that limit is being set or figure out how to change it.

I've attached a stand alone file, test.html that illustrates the problem.  (I had to name it test.txt because it wouldn't allow an html attachment). I've also got a screen shot.  I've searched through the CSS files but can't find anything with 315.  Can anyone point me in the right direction?

Thanks