What are the typical main track drive motor sizes used in 1/16 RC Tanking?
380's? Or is it more drive system/tank dependant? (Or user upgraded?)

All the none driving functions are 280 motors, and the stock tank all comes with 380 motors for driving. You can upgrade to more powerful ones (still have the same 380 form factor) off brand ones, or Promax 400s(which I believe are just higher end 380s) and also 480s. 480s are longer cam and you can't fit them in some of the tanks. I couldn't upgrade my Challenger to 480s, so I just end up with the black after market 380s.

There are now some gear/motor combo where you get everything in one package. My Leo has an  enclosed gear box with motors and they are suppose to be "550s" ?!?!?! Don't know since I never open it up.

The 400 motors are identical to the stock motors , except with a little tweeking, Promax probably doesn't take the motors apart since they are sealed motors , they take the best ones out of a batch that they dynoed, rezapp the magnets to match and strengthen them , as a former slotcar racer , I still have access to a zapper and small dyno , I recently got a stock motor to make more power than a 400 , by way of zapping and matching magnets , centering the armature in the can , and advancing the commutator on the armature for more torque but less top end , yes it worked great, but to build 2 identical motors is a different story , I'm working on that

I'm very particular to buying motors and I get a very specific type and rating, I'll have to dig it up but I've posted plenty of times about them... nearly double the RPM and a buttload more power too. I doubled Bud's Pershing performance with them..! 

Here you go, where I got mine. I still have 4 pairs remaining, I can install them for $20 
http://www.aliexpress.com/item/RS-380SH-4535-DC-Motor-for-Vacuum-Cleaner-6V-18000RPM/32406585604.html?spm=2114.01010208.3.26.rDKMqj&ws_ab_test=searchweb201556_10,searchweb201602_3_10017_10040,searchweb201603_1&btsid=f029d63d-d183-4b06-ac03-62a62d218399

These are the "real" 380s !

I just ordered 10 more so anyone wanting a significant power boost, ask me

I'll probably need to borrow your tank as I need to solder, gear pull, and probably clean up your messy as hell wires while I'm at it..

...also airflow & Clark / Whatever cooling becomes more important.

hey folks why not buy the promax directly from the manufacturer?
here is the link. http://www.maxxprod.com/mpi/mpi-10.html
Acc341 for the 400 motors and Acc348 for the 480's
The biggest difference with these motors vs china stuff is they are the best of the batches and they run higher rpm than standard stock 380s(more reasons they are better but not getting into it )
Torque doesn't mean much when running tanks because your power comes from the gears and not so much from the motor.
Higher rpm will increase the torque more than a stronger motor. 
Its all in the gear ratio see the link and pay attention to the last paragraph. 
http://www.teamdavinci.com/understanding_gear_reduction.htm

Yeah , i totally forgot about the gear reduction while playing with the motors , I didn't realize how cheap these motors were to buy , after reading the last thread , it got me thinking if anybody makes a 64 pitch gearbox over the stock 48 pitch that every tank comes with , it would be an expensive gearbox , but you could hear a pin drop beside it running , even without bearings