The parachute subapp computes the needed parachute size or descent rate for a given parachute size.
Set the mass of the rocket with the bottom slider. Choose a shape (spherical has more drag). Select what you want to compute, parachute diameter or descent rate. Depending on what you selected, one of the top sliders will be visible, the one being calculated will not be visible.
Use the slider to set either descent rate or parachute size, iPowerRocket will calculate the other.