Cast net recommendations for my son (14 years old, 5'6" tall)

[yigodiver]

The question would depend heavily on the experience level and some stamina in the conditions you have indicated.
Here is an explanation of my experience: Last fall while at the Weirs at Marsh Island, we were catching some shrimp, only my son 13 year old, and about 5'2" and I were casting and the wife and daughter were observing. The net I was tossing was 8ft, my son was throwing a 5ft, we were doing decent on shrimp but not busting the sound barrier. A rope was spotted floating in the tide movement and it appeared to be a cast net rope, I am a scavenger of sorts, so I got a fishing pole and snagged it and sure enough it was a cast net, brand new with tags still on it!!! I believe 3ft. My daughter then had a net, we gave a short course on throwing it, and she began wearing the shrimp out to our 3 to 1 and she is 10 years old and about 4'8", or shorter. I like the cheaper Wal-Mart nets they work and not as painful when you snag them up. We also catch lots of shad.
Last piece of advice, learn to use in the yard on the grass, can refine the technique, bigger net=heavier net, but sinks faster.
Good Luck!