I found a chart to help with sizing:
http://www.the-house.com/site/info/i...-05boards.html
Get a used quality board. That's one thing you will get with renting. You can go out and try a few different brands, and see which one you like the best.
Other than that, read a lot of reviews.
Since you are already a skier, you will pick up snowboarding faster than someone who has never been on the snow. So if you get a quality board, then it may last you a little bit longer.
Good boards from when I was riding a lot:
Ride, Option, Burton where probably the top 3.
Burton does a really good job. They always seem to be at the leading edge of technology.