It's going to be about 1' square and almost fully metal.
Tin? Lead? Copper? Titanium?
You might consider modifying a large servos like below for continuous rotation to drive the tracks.http://www.hobbyking.com/hobbyking/store/__18743__Vigor_VSD_11YMB_MG_HV_Extra_Large_360_Degree_Winch_Servo_0_75sec_40kg_150g.html
I'm not sure how far you'll get with just the chain - take a look at this site for ideas on treads and other things.
I would appreciate it if we could keep focus on the question at hand. It hasn't been figured out and I'm settled on the design in that aspect.
So what have you done for yourself and your project so far? No body knows what your design and building capabilities are. What kind of bearing will you use? Your best bet is to research similar bots to what you want to build, and go with motors those bots use.
So the motors I was looking at have a max torque of 619 oz-in (or 17.5 kg) @12VDC. And that servo is 40 kg torque.