Hi Apoch,
The 100K pot has only about 6V across it so dissipates only 0.36mW. A standard 1/2W pot is fine.
Select a Mosfet with a max current spec to match the stalled current at 12V of your motor.
If you use the circuit as a DC lamp dimmer, select a Mosfet with a max current spec to match the cold current of the lamp. It will be about 10 times its lighted current.
You will find that N-channel Mosfets are less expensive than the P-channel one shown. Connect an N-channel Mosfet with its source to ground and the motor connected to its drain and to +12V.