Yup, skill/practice is the ultimate game changer. As for best pitcher size it depends on how powerful your machine steams, how much milk you want for your drink(s), and how much milk waste you find acceptable.
Let's first assume your machine provides all the steam you want. If you're making a 5oz cappuccino or flat white, you can probably get by with a 12 oz pitcher, but it'll use up just about every drop of the milk to pour the cup. If you want a little wiggle room, a 16oz pitcher will give you that, but then you'll have a couple ounces of milk waste. If your cups are 7oz, you'll definitely need that 16oz pitcher, and if you have 10 or 11oz cups, you might need to got an even bigger pitcher. Likewise, if you intend to pull a couple of shots for small cappas or cortados, you'll have to decide if you want to steam twice or use a large pitcher. Of course, it also depends on how thick you steam the milk to, though if it's too thick you can't pour art well.