First of all, it all depends on your preferred riding style. (Since you are a beginner and probably don't know what you would like to do/be good at, try some rentals or demos first before you buy a board)
For freeriding I would recommend around 161 cm
For freestyle I would recommend around 155 cm
--> Give or take 5-9 cm on each to your preference. It's all about comfort, so feel free to choose anywhere between 152-165 cm but since you are still growing I would recommend that you choose a mountain board around 157-8cm.
For me, since I'm lazy I just measure my freeride board to my lips/nose and my freestyle board just below my chin.
--> Longer boards:
Pros: Stable @ higher speeds, better in powder
Cons: Harder to get on rails/spin (park stuff)
--> Shorter boards:
Pros: Easier to spin/get on rails/boxes (park stuff), easier to maneuver
Cons: May catch an edge easily; not as stable @ high speeds
Also, remember that different kinds of board shapes ex. camber/rocker etc. can affect the way it rides, so for example if you would like a shorter board but don't want it to catch an edge easily, you could get one with a gullwing camber. (:
+The flex of the board is a very important factor too.
Pros: Easier to learn, on, easier to turn, easier to "maneuver", better for park stuff
Cons: Washes out easily at higher speeds, a lot of chatter
Pros: Easier to initiate turns at high speeds, more stable to land on when going off jumps
Cons: May be difficult to learn on, stiffer makes it harder to turn at first
But again, it's all about preference!
PS...If it's your first board, you might not want to invest in a super expensive one just in case.
Take your time when picking it out! (Try a rental/demo one first and see if you really like the sport though...you would't want to waste 400 bucks and not use the board after one try...)
Hope I helped in some way or another haha
and all you're thinking about and dreaming about is just snowboarding the next day