Do you really mean "empty page" (that is, none of the portlets, nor the portal header or navigation tabs are showing) or do you mean that your portlet's window is empty?
The portlet window will expand to whatever height is necessary. IN other words, there is no vertical height restriction.
Have you asked the browser for the HTML source of the displayed page? That might give some clues as to what is going on.
The next thing after that would be to debug your portlet - it is probably causing this problem.