How long should my snowboard be (Burton)?

hi i am buying a Burton Troop but hesitating between two sizes - 146 or 151. i've tried the 146 and liked it. but when i went to the store, the staff told me i should get the 151 if i want to do powder and don't always stay on groomed trail. i am 5'2" and weight 120lb. the 151 board is a little above the tip of my nose.. i am afraid it'll be too long and not as easy to maneuver....looking for advices.. thanks!!!