To open the parachute door a servo is used. The one I am using is "small", and runs on 4.8 to 6V. Servos can use a lot of current, so it is not a good idea to run it off of the Uno board power supply, better to directly power it off of a battery. I briefly ran it on a 9V battery to test it out, and it survived, but I will have to get a 6V supply (probably 4 AAA batteries).
"Small" should be fine, as in this deployment design the servo just pulls a pin that allows rubber bands to open the parachute ejection door (not a lot of torque or travel required).
Coding is again easy, as someone else did most of the hard work. The "sweep" servo example that comes with the Arduino software worked the first try for me, and is easy to modify.
The timing of opening the door is controlled by the Arduino. I haven't decided the exact logic yet. Some options are: 1) Apogee (maximum altitude), 2) time after launch (simple), 3) a low altitude on descent (results in less drift while parachuting, makes it easier to recover the rocket, but riskier for deploying in time to be useful), and 4) any time the rocket starts to descend (might help limit the damage in errant launches).