My son said the sweatshirt isn't really waterproof, but water repellant. Althought he thought you'd be ok if the snow isn't slushy.
My son is 5'4", very slim and he wears a medium. He thought a large might be big enough, but if you want a really huge and baggie look it could work. He is a skier though, so I know snowboarders like bigger outer ware.
This item is water resistant, but not water proof. This item has been treated with a DWR treatment. (Durable Water Resistance)