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Because you decision is in concrete, I would recommend 8'6" x 10'6" to cover all of your bases. this leaves adequate room for steps and maybe a side bar and always leaves the option of a gazebo. Better to plan ahead and be prepared just in case. Otherwise, minimum size would be the dimension of the spa plus 6" around. It really depends on what you will want to do in the future, but 1 or 2 extra yards now leaves the options open.