Many of the dimensions are stipulated in the FIA rules... for example minimum monocoque width, opening for the driver to get in, position of the driver's feet etc... you can download them from the FIA site. The rest are determined by the design of the rest of the car... I have a very simple monocoque in my solidworks model.... compared to 2009 monocoques they should be longer and wider to accommodate the new bigger fuel tank
see this thread for a shot of my model (second pic down shows the monocoque) (since I made this I have subsequently widened it to accommodate a wider fuel tank):
viewtopic.php?f=12&t=7764