[KenWittlief]

do you mean when you turn the pot up and down the motor goes faster and slower, or to you mean the motor tracks the pot postion, like a servo does?

you can get large pots that will handle several watts to control a small DCmotor, like you would have on a train set

if you want the motor to act like a servo, so the motor turns and points like the pot does, then you need a second pot connected to the motor shaft, and you need a linear power amp and an op amp circuit

the control pot would be compaired to the position of the feedback pot (attached to the motor shaft) and the error signal would be what drives the imput of the power amp - the output of the power amp makes the motor move until the two pot positions are matched.

This would be a PID feedback control loop.