Although I am supposed to be working I am going to take a break and see if I can give you some useful information.
1. I personally dislike the S-10 frame route so I suggest selling it
2. There are many discussions with photos on how to build a frame here. However, here are some pointers.
Learn how to weld, and I don't mean gorilla weld. If you cannot make a decent weld don't even start your own frame. You should also know how to create a good weld joint, if you can weld but don't a T joint from a corner joint you probably don't want to go the build it yourself route.
That aside, most folks use 2" x 4" tubing with .188 wall.
As for dimensions, you will need to figure those out on your own based on what you are trying to do, frame width, length, etc. are all governed by what you want and what you need. i.e. SBC engine is so long, plus tranny, plus radiator, then you need to figure on the cab, rearend location, ride height and much much more.
If you are new to this go read everything you can and then make a plan.