You won't like the width. Both of mine are 5' x 16" and at times that is wider than I want it. If you use 4"x4"x1/4" on the main rails you won't need the trusses. Mine is 3/16 and I wish it was 1/4", it would take some of the "ring" out of it.
Here is a pic of mine, disregard chassis.
Three legs on each side and three main crossmembers welded in, the rest of the crossmember float so I can put them where I need them for whatever I am working on......or get them out of my way. The legs are built in two sections so I have two different heights available. Casters or adjuster pads are usable at either height.
Make it as simple and as universal as you can, that will ensure that it stays useful. Also remember that you will want to work under it so leave enough room that you can.
Another note on the truss work, the more you weld on the main rails the more likely it is to pull out of level.
One more thing, leave the ends open. My welded in crossmembers are 2 feet back from the end of the mainrails, someday you are going to want to put the motor in the chassis and that crossmember will be in your way.