depens alot on what you want to do with the board, what kind of board it is and what you prefer. but i do think if your planning on doing park then you should het a 152, im 165lbs and ride a 156 allmoutain and id prefer a 154 to be onist. but its up to personel prefrences aswel.
yes a shorter board will make olie-ing easier, and nose-tail presse4s aswel and steering aswel.. basicly a longer board is just more stable..
and yes you will most deff notice 5 cm, some people say they even notice 1cm.... im not one of them tho.
you will notice 5 cm