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How much stuff are you going to keep in your room besides your table? You need enough room to walk around your table.
If your table is 3 by 7 you will need at least an extra 3 feet on either side just to walk around. That means you need at least a 6 by 10 room. Now if you have extra stuff in the room you need extra space for them as well. Figure 1 to 2 feet extra. You can however add that to the 6 ft side. Now it becomes a 7 by 10 room or an 8 by 10 room that you need. Depending upon how big you are you might be able to get away with 2 feet on each side instead of 3 feet. You can add 1 or 2 feet in any direction on top of that.
Any bigger though and the room starts looking too big. A bigger room also can make things awkward as you may have to leave the client move around to much to get hot stones or anything else you might keep on shelf or counter space.
that sounds about right if your looking for the reasonably smallest room you can have. It really depends on what your going to use the room for. You might think about having some small rooms, and a few a little bigger. That will give you some flexibility if you change your mind about things later on.