Actually, you don't really need the resistors. They're only there so you can sense the current flowing in the motor windings. If you're not interested, you can just connect those terminals to ground.
The ones in the product are 0.5 ohms so at a maximum current of 2A they would dissipate 220.5=2W, so they've been sized for 3W (reasonable I guess). Unfortunately the datasheet specifically says not to use wirewound resistors (highly inductive) and that's exactly what they used 
(it looks like)
If you want current sensing I'd suggest 0.1 ohms for lower power dissipation so you'll get voltages in the range of 0V (0A) to 0.2V (2A) and they will only dissipate 220.1=0.4W so a much cheaper/smaller 0.5W resistor will do. You can find those at any distributor, Digi-Key, Newark, Mouser, etc.
--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons