I am 5 foot 9 and i am going to buy a snowboard, but I don't know what size i should get?

If your planning on going to the park definitely go with a shorter board. I am around 5'9'' as well and I use a 148. If you don't wanna take my word for it at least try renting a smaller board and be amazed at how much easier it is to maneuver.

The size of your snowboard is really based on weight. The snowboard can not tell how tall you are, but it knows how much pressure your applying to flex it. The length of the board matters for reasons of control and stability. The shorter the board the easier it is to maneuver, and the longer the board the more stable it is going to be at higher speeds and steeper terrain. Every size snowboard will have a different weight range. The longer the board the more weight the board can hold. If your shoe size is 14 you should definitely purchase a wide board. You will have severe toe drag on a regular width snowboard. The quality of rental equipment is significantly less than what you will be purchasing, and when you rent at a mountain they will usually downsize you to unsure that your one day experience is enjoyable. I would need to know your weight, ability level and how many days you plan on riding a season to make a specific recommendation.