Bit of trans-tasman advice (I'm a pomme living in NZ), I haven't ridden the boards you are looking at but my friend owns the raygun, he is a little more park orientated and hasn't had any dings out of the usual ones picked up on the lifts, boxes and rogue rocks.
In terms of size I am a little heavier than you (85kg) and ride a 157.5 without any issues, if anything its too small for any hard charging but that isn't my style of riding. If you are happy on a 154 then stick with it, but I would suggest going up to the 156cm as it will give you a little more edge as your confidence grows and is still plenty small enough to spin and play on. Don't worry you won't suddenly be flying out of control jumping up a bit in size. If in doubt beg, rent, borrow and steal to see what works (don't steal unless you are taking from the rich to give to the poor)
enjoy, let me know if you guys get any snow over in Ozz, we are struggling this season in NZ