I'd think a beginner (adult) would probably be using something a little bigger, say 5m and up? I know I did (was recommended a 6m), and still do in light winds just for the practice... light wind gybes, tacks, etc.

There are probably a few reasons why beginners are advised to use a small sail. First is the ease of uphauling a small sail compared to a big one. Second is the lower weight of the rig. Third is a beginner normally has not the ability to counterweight the rig so you need a rig with little power. I am sure there are other reasons too.

I quite enjoy sailing in light winds and little sails. Its nice to try some sail spins and stuff like that. I've only done it a few times when I was teaching my wife to sail. Thats all finished for now as its too cold for it to be much fun for her.

Gotta remember though during the boom years of the late 70s and early 80s most people learned on boards like the TC Beecher, or the Windsurfer, and a storm triangular sail which is about 4.5 square meters. I know I did.