Deciding on a parachute seemed a little harder than I first thought. It needs to be big enough to slow the payload decent so it can be recovered intact. At the same time if its to big the payload will drift a lot further on its decent and potentially take much longer to reach the ground.
To assist with the parachute decision most manufacturers provide a guide to the decent rate based on the weight of the payload. Some parachutes aim for a decent rate of around 5m/sec. Spherachutes lists it chutes based on the payload weight and other websites provide similar info.
For my project I went for a 48″ or 1.2 metre chute from Spherachutes.
This seemed to fit the estimated weight of my payload of which was still under construction so I hadn’t determined the final weight. Spherachutes charged a small additional fee to sew some straps at the top of the chute for the balloon to attach to.