How big of a snowboard do I need?

I am 5' 8" and weigh 145 pounds. I have around $200 to spend on a halfway decent board. If I can't find a cheap board I'll have to rent the entire season and I don't really want to do that. I know the good brands so that isn't a problem. I know all the sites, I just need to know a good size board for me. Last year I tried riding a 163 but that did NOT work for me at all. I would like an all-mountain board because It's only my second year so I'm not pro. Any help or online calculators would help extremely.