hello newrider,
I have two boards, one that is under 40'' for freestyle and one that is 46'' for racing and bombing down hills. Of the two boards, when i take people with me to mountainboard, the taller riders feel more comfortable on the 46'' because it is easier to control. Generally, taller riders feel more comfortable on longer boards.
I would find out how long the mountainboards are that you have been riding and find the one that you feel most comfortable on.
At your size, whether you are doing freestyle or just freeriding, I would say the Blue Earth 39'' B or C series would be the best choice, anything under 39'' in my opinion would be too small if you are looking for a mountainboard that can do both freestyle and freeriding.
Other companies often make their mountainboards that are designed for both freestyle and boardercross to be around 40'' long.
Hope this helped.