You do not have to specify a fixed height but a height in general or the default height (see your screenshot) is applied. Relative heights work, too:
<div id="pipeline_graph" style="height: 80%"></div>
That is how modern browsers work. The modeler can only occupy space that is given to it by the browser. You as the user have to make sure the container element has an appropriate height. The modeler will fill it up.