Outdoor Palms in PA Landscape

[Artistic Impression]

I am looking to add some Windmill palms to mine and a couple other landscapes. I have considered adding a few needle palms too. Now I have heard that they can handle the snow but, how big should I buy to help insure that they make it. I imagine the more mature they are, the hardier, and more likely hood to survive. If anyone has had any experience with keeping palms outside year round in zone 6 or further north, I’d love to hear your advice.

Thank you
Jim