In order to determine the right chute size, you need to "close the loop" on your initial rocket design. This loop looks something like this:
(Chart showing rocket mass, chute calculations, adding in chute mass, recalculating target altitude...)
Fortunately, after the first couple of runs, it should converge. To properly size chutes, you want to consider the terminal velocity stage of descent. You want your rocket back at the end of the day, so all you need is a chute that will make it descend slow enough that it won’t break. If you go much slower, you’ll drift for miles. The generally accepted “safe descent speed” is 20 ft/s (~6 m/s), but can be as high as 25 ft/s.
One driving factor for the descent speed is the shape of the fins. Typically, when the motor section descends, the fins are pointed down. The single most common failure point on descent is the fins – they will often break upon hitting the ground. If you have pointy fins, consider a slower descent speed.
Once you know that, use the lovely drag equation, and solve for Area of the chute. Include the mass of the chute! (or an approximate value that’s close.) (Assume the sea level density of the atmosphere is ~1.2 kg/m^3, and the Cd of a hemispherical chute ~1.7.)
(Picture of Drag equation, substituted)
Once you have the area, find some chutes (via specs online, or in lab) that match that area. If the mass of the chute is significantly different than before, do the math again until you converge.
Aaand yer done! Happy launching!