If ever there was a topic that seemed straightforward, but ended up being fraught with bad, incomplete, and misleading information, then parachute selection is that topic. There are many ways to go from here, and many of them lead frustration and confusion. One day we may go into all of the technical details of selecting a parachute, but now is not the time.
Parachutes basically do their job by being large, light objects that have a large amount of drag when traveling through a fluid like air. This drag is created by the difficulty the air has moving out of the way of the parachute as it moves through the air. The larger the parachute, the more air that has to move, and the larger drag force.
There is something that you understand intuitively about parachutes, drag, and friction in general, but you may not know it explicitly. The fact, to which I am referring, is that friction (drag) always acts in the direction opposite to the direction of travel. Ta da!
OK, so we understanding how parachutes work, but that leaves a lot of details out, like what shape and size parachute should you should get? A descent rate, on the low end of our window, would be about 7 mph  (3 m/sec, 10 ft/sec). I say this is on the slow side because this is for recovering amateur rockets, which can go very high, but in almost no case are they going nearly as high as our high altitude balloons. Since long-distance travel and nearly hour-long returns to Earth are not part of amateur rocket launches, they will likely tend towards conservative descent rates to keep their hardware safe.
With high altitude balloons there is a tradeoff between getting your payload back safely (a slow descent) and getting your payload to land in the same state that you launched it from (a fast descent). With this in mind, consider the following process for selecting a parachute:
Go to http://www.rocketreviews.com/parachute-size-calculator.html and enter the weight of your entire system without the parachute or balloon.
Pick one of the descent rates and corresponding parachute sizes. Factors to consider: How fragile is your payload? Will you be launching lighter or heavier payloads in the future? Are you more worried about payload damage or driving all day?
Pick a parachute of a similar size from http://www.the-rocketman.com/recovery.html, which are specifically designed for high altitude balloon missions. They claim that their parachutes’ clam shell shape improves stability, and I tend to believe this is true based on my observations.