Yo... size does matter (when it comes to snowboarding). Like stated before, if you're gonna ride powder and bomb hills, you're going to want a longer board. If you're mostly park, you'll want a shorter board. Also, you have to watch with weight. I'm 5'4 and about 155lbs and I ride a 152.. and just about mostly park (with the occasional glory ride). I've talked to numerous people and they said a 148 I believe it was, was not too practical being my weight was a little more.
You seem to be just about my size, so I would definitely suggest a 151-152. It really comes down to preference too and what you feel is right. I will use my board for example.. I ride a 152 Rome Agent... if you go to their website.. they recommend that the weight for a 149 is between 85-140lbs and a 152 is between 110-165lbs.