First & foremost, it's been my experience that one learns the foundations of sailing in small boat better & faster than a large one, but I'm not an expert. That said, no expert or experienced sailor I've talked to has disagreed with me yet
Secondly, 16ft is the dinghy size I've learnt (and still am learning) on. Cats in that size are fun, but there are some things you don't learn on them. Flying a spinnaker being a good example. Different physical skills in sailing a cat that size and a mono too. I get a sore back & legs pushing myself physically in the cat, but sore stomach muscles in the mono.