Best brand for dresses! I've tried all Canadian brands at least for bamboo/tencel modal and the skirt of these are the nicest for flow & twirling. They have really nice length as well, I find other brands are bordering on "tunic". You do NOT need to size up unless you want even more length. Stick to true size, or size down for shorter mid thigh length. I find true size hits at the knee but not below.
We sized up here and it still works its below the knee but we are walking full-time (neck hole isn't huge either, it's structured well)
Bumble nums dress in 18/24 (but usual size to buy 12/18) at 22lbs & 30".