Yes, it's very frustrating sometimes, using the 3D Box tool. If you draw outside the page border, it doesn't even look like a box, sometimes.
Like Lazur suggested, start by drawing a "box" inside the page border. After you have a "box", you can adjust the vanishing points, which will help you to move the box outside the page border. Also, see the manual for controls
http://tavmjong.free.fr/INKSCAPE/MANUAL ... Boxes.htmlIn my experience you need to have a fairly good understanding of 3d geometry, if you want to use the 3D Box tool for any kind of precise drawing. The vanishing points and etc can be confusing.
One thing I've always wished for, with that tool, is a 'restore to default' button. Not sure if I ever made a bug report for it.