Aman is right, if there is a failure of the transistor, you could knacker the micro. Peter is right, 50K will be more than enough. The base resistor is chosen to make sure that the load current saturates at the minimum gain of the transistor (Hfe), so if your load =10K on a 5V rail, you will get 3V across it saturated, 3/10K= 0.3mA Base current is Hfe times (lets guess at Hfe= 100) less than the load current, so base current= 3uA. Drive to the base resistor = 4.5 volts say, saturated base-emitter voltage =0.6 V. Therefore 3.9V developed across your base resistor when 3uA flows thorough it. Therefore base resistor= 3.9/3e-6 = 1.3 Meg ohm !

Make sure that the output impedance of this source isn't going to be too high for your following circuit.

Steve