Control 5V DC motor with Arduino
The resistance depends on the motor's maximum current consumption and the transistor's h(FE) - the ratio between collector and base current. As I can see in the datasheet (http://www.datasheetcatalog.com/datasheets_pdf/T/I/P/1/TIP132.shtml), TIP132's h(FE) is at least 1000. Say your motor consumes at most 2 A (which you have to find out yourself, either from its datasheet or block the rotor and measure the current consumed), so the microcontroller has to source about 2 mA to drive the transistor.

The transistor's base-emitter voltage should be about 1.5 V (it's a Darlington transistor, that is, two base-emitter junctions are connected in series, dropping about 0.7 V each), so the resistor should drop about 3.5 V with 5 V supply. Having that we can calculate that its resistance should be: 3.5 V / 2 mA = 1,75 kOhm, so 1.5 kOhm would be suitable in my case.

Again, check your motor's maximum current consumption and re-calculate the resistance. You may also find an interesting option in replacing a BJT with a suitable MOSFET (an IRL3202 could be a good choice). A MOSFET has lower "open-state" voltage drop than a BJT, so it allows to deliver nearly full supply voltage to the motor and dissipates less heat.