change the size of 3D camera navigation view

Greetings, I'm working on an irregular size 3d composition (720x2048) which makes 3d navigation a pain in the neck! Does anyone know how to set 'Custom View 1' size or aspect ratio differently from Camera View? I'm hoping to be able to navigate in a standard 16:9 view while keeping the output preview on my second monitor at 720x2048.

Thanks