What John and AlfaBrewer suggest, however. You'll need to take note of the 'expected' winds where you will be launching. Ground level, as well as upper level where the drogue will be deployed. I say this due to a recent experience I just had in Argonia, Kansas. I launched an 8.5lb rocket with a 15" drogue and a 30" Recon main. It went up to about 2,200 feet. The ground winds were about 16mph. Not sure of the upper level winds. That rocket came down and was recovered within 300 yards of the pad. I 'then' prepped a 10lb rocket that went up to about 4,800 feet. The winds picked up to about 19mph by launch time. Not sure about the upper level winds on this one, either. I had a 18" drogue on this one, and intended to use a 60" main. Due to the higher winds, I pulled the 30" Recon out of my 7lb rocket and put it into the 10lb bird. The main worked just fine and brought it down slow, albeit. the 18" drogue got caught up in high winds aloft (maybe some thermal activity too). Darned thing wouldn't come down, on the 18" drogue, till it drifted halfway to Nebraska! Once the upper level winds, etc. let go of my drogue, and the rocket FINALLY started to come down... everything went fine and the rocket quickly came down on the main, as hoped in good order.
I know this info is a bit vague, and probably not much help, specifically. Take it for what it is. The hardness of the ground is a major consideration in main chute size, but watch that drogue in high upper level winds, especially if its all brown dirt below. "'Thermal' activity might make for a long flight/drift even with a small drogue."
