Desired descent rate.
That figure would take into account things like field size, anticipated altitude, rocket profile and weight. Generally, the consensus is that around 60fps to 100fps is about right. If your rocket has a profile (big fins, lightweight construction,etc.) that is somewhat draggy, then you probably want to aim for the slower end of the range (bigger chute). A very slim or heavy rocket can benefit from having a higher descent rate and smaller drogue.
While not super-accurate, the Openrocket SIM does a fairly good job of estimating descent rate.
Obviously, once you decide on size, you still have to watch how your rocket descends and tweak as necessary.
On every DD rocket of mine, I've ended up adjusting the size up or down based on the falling profile when it's under drogue. I've never found any rule of thumb, calculator, or simulator that really works well. They get you in the ball park, usually.