I’ve done this kind of planning few times in my career thus far…
Sheet metal forms are traditionally mocked up using 3-5mm thick foam board that comes in 1200mm x 2400mm sheets. Its quick, cheap and gives enough information to update the 3D database that will accompany each concept as well as plan for any intricate fabrication details. In recent history, large caps, fittings, interface bezels, knobs, feet, handles etc. were usually made from urethane foam which comes in 1200mm x 2400 mm sheets. Now in 2015, these parts are 3D printed and then mated with the larger foam board volumes.
Any workshop planning should entertain a few initial space considerations first before filling it up with fabrication tools. This will insure the importance of producing high quality work. Material storage space, work space, lighting, ventilation, noise control, tool storage space and control/storage/disposal of waste are all areas that should be discussed and considered before making any investments in more expensive fab tools.
If you plan to prototype in actual sheet metal, the same spacial considerations for the above can be exercised again. However, a hard materials metals lab is far more different in its specification that that of a soft materials lab. If you plan to model in both materials, you need to double your space requirements in order to keep separate each material and its accompanying fab tools (i.e break presses, grinders, welding gear, cutting tools etc.)
Good luck…