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Run the modelviewer once, then go to "My Documents/HPL3" and open "modelview.cfg" with a text editor. In that file change WindowSize="1024 768" to be your desired resolution, for instance: WindowSize="1600 1000".
To help find why a mesh is broken or invalid you can use the Show bad vertices checkbox. It will draw a circle around every invalid vertex on the mesh. The circle is color coded to show how it is incorrect.