I have a 13 year old student, she's 4' 11". I introduced her to a Back Plate & Wing system and a long hose configuration with necklaced backup. The long hose is 7 feet. She was apprehensive at first with the long hose but after the first dive with it, she loves the setup! She just loops the excess long hose length and tucks it in her waist belt. Deployment and re-stowing is a breeze. No problems whatsoever.
In an air sharing situation, the 7 feet long hose is the best choice, even in open water. The space and maneuverability it gives you and the receiver make egress more manageable and safer particularly if an underwater swim is required to go to your exit point.
My students ask me if they should get a 5 feet or 40" hose. I strongly recommend the 7'. It may seem long on paper but they appreciate the extra length when they actually have to use it for its purpose even in open water. Plus, no changing of hoses if the dive trip includes an overhead penetration. Pre or post dive, just coil it on itself and clip to the right shoulder D-Ring. Easy.
For those who think 7 feet is too long, please try it first in a simulated OOA practice with swimming to an exit point. You'll see the benefits immediately both as a donor and receiver.