First measure the winding resistance of the motor - is there any current, voltage or resistance information on the
motor's ident plate?
The ULN2003's power dissipation for a particular current can be worked out from the graphs on the datasheet
(saturation voltage is fairly high, its a Darlington device) - if you're pulling more than a few 100mA it will need
a heatsink and only goes up to abs max of 500mA per pin anyway.