How big should my parachute(s) be ?

  Ounces Pounds Grams Weight of the rocket including motor hardware, but no propellant.  You can find the motor hardware weight here.
Descent rate in feet / second. A good rule of thumb is 15 feet per second for main chute descent rate, and 50 feet per second for drogue chute descent rate.
Launch site elevation

Number of Parachutes (Assumes all chutes will be of equal size)
1    2    3

Type of Parachutes
Round    Hemispherical   X-Form

You need inch diameter parachute(s)