The idea was to use a single sheet of hardwood plywood, and some basic hardware to build this desk for between $75 and $100. It could probably be built for less if you changed some of the materials.
Unfortunately I didn't document the build process at all, so I only have pictures from several years after it was built, and some pictures of a much larger version that I built later. I've included pictures of the other desk (a similar design), and the completed desk as it is today.
Step 1: Planning
I laid my desk out on a sheet of graph paper using a 4 squares per inch rule.
Step 2: Planning: Legs
So the height of the legs is measured off of the side of the plywood, and then split in half to form two legs.
Step 3: Planning: Top
The wood left over from this cut is used to provide a backing panel for the desk to strengthen the design.
Finally, how long should it be.
The simple answer is "whatever is left once the leg portion is removed", but you can always shorten it to fit your needs or space. At most this will be around 72"
Once you have the basic measurements, you can calculate how much support wood you'll need as well.