Link to this javascript in the head of the iFrame content document (the child document).

// JavaScript Document

if (window.parent.screen.height >= 700) 

{ document.write('');
}
	
else {

document.write('');
}

Of course this could be used for redoing the styles of the iFrame contents based on any other property of the parent document as well. For example you could say

Parent.document.body.some.property= 'a value"

window.parent.screen.height