Based in the USA though you may find our suppliers a little expensive when it comes to shipping, this is an exclusively (almost) european forum. Thats good though actually, a fresh european view may add to any US advice you get.

Id echo the above, tank tracks are not the ideal first project, but did you mean tracks or did you mean a robot that drives like a tank, ie, can turn in the spot?

Robot sized vary conciderably, but Id advise you waste as little space as possible inside the machine. A bar 1 foot long could have been twice as thick if it was only 6 inches, over simplifying a bit.
you will no doubt have more help than you can ever use as people here are usually very kind offering advice, but if you do have any specific questions e-mail me
james@jamesbaker.f9.co.uk
and Ill do my best to put you in touch with the best person.