Are you wanting to make this yourself our are you looking for the dimensions so you can take it to a shop and have them build it?
If your doing it yourself then I would recommend finding some designs that are out there that you like and start from there. Draw out a few ideas and then get started with cardboard. One trick I learned is to use bobby pins (like you use for sewing) to hold your pieces of cardboard together while your designing. I wasn't trying to be a dick with my first response but there aren't to many DIY home shop guys that have the computer software our the machines to use CAD. The process I mentioned is the way I make all of my stuff. I take all the little things that Ive seen and combining them into one to make something of my very own.
Heres a link to my front bumper build, hope it can help in some way.
http://www.texasjk.com/showthread.php?3800-My-bumper-build
And as you can see, Cardboard Aided Drafting works great