The body of the robot is framed with aluminum angles from Home Depot. The angles are 1" x 1" with a thickness of about .040". There are three lengths used: 36 inches, 8 inches and 13.75 inches. There are 4 pieces of each length.
Your size is up to you. I chose this size to make the robot tall enough that an arm mounted on the side at the top could reach onto a table. It is wide enough to mount my laptop on without it overlapping the sides. And the front to rear dimension is just picked large enough to make room for the battery and for shelves wide enough to hold at least the microcontroller board. Note that the microcontroller in this picture is a prototype model.
Each corner is formed by just
putting the three pieces together and attaching them with 1/4 inch bolts and
nuts. Generally, the bolt length is 1/2 inch, but I used longer bolts on
the bottom to pass through the platform for attachment.
Watch out when drilling holes for the bolts that you do not do a precision job of locating the hole exactly in the center. If you do, the ends of the bolts may hit each other in the middle. Of course, you can look for some 3/8 inch long bolts; or just locate the holes a little farther from the corner to provide more clearance.
Those of you who are structural engineer types may notice that a rectangular shape like this is not very dimensionally stable. It could be pushed over forward or sideways by just pivoting the corners. However, I've found that (so far) the three screws in each corner hold it quite well. If you ever have any trouble with leaning, some gussets could be added.
Of course, this is just an example of how a body may be constructed and is the method used for Mabel and Rocky. Bruce's robot, Leaf, demonstrates that the body may be built with any construction technique you prefer. Leaf's body is a water conditioner salt tank. Looks very R2D2!