Depends upon your environment and your species of palm... If you live in Hawaii, you can pretty much plant out anything at any time. If you live a pretty nice climate (so Cal or Florida) and you want to plant out Washingtonias, again, you can pretty much plant them out at any size. But if your palms are marginal, or you live in a marginal climate, the bigger they are when you plant them out, the more likely they are to survive. Most palms grow faster in pots if kept in a greenhouse. Pots also make watering palms easier and likely to get all the roots wet better, so will often grow faster (again, this can vary depending on your soil, your climate, and your palm).

I'd agree. After you plant out palms they can take a bit of time to readjust to the change. Small ones often adjust faster, suffer less root disturbance, and sometimes will grow faster. It's a bit of a juggling act between species, climate and the environment you provide.