Make sure that when you do your base, regardless of what it is, don't forget the steps. They also should be on a firm, solid base.
I would do 1 1/2 feet more in each dimension, that gives you 3/4 of a foot all around the perimeter so no grass or weeds grows up next to the spa cabinet. It also looks very clean when trimmed off that way.
good luck,
hottubbe...