How much space must be allowed for the cover and lift system? If the HS spa is 6'6'' X 6'6", how much space is needed to lift the cover?
This will depend upon the specific cover lifter system and how much of the cover you want to drop behind the tub. Some people want cover lifter to sit as low as possible when open to provide the best veiws, which requires more clearence between the tub and any walls.
Others prefer the lifter to sit up higher when open to provide privacy. This requires less clearence between the tub and any walls.
Each lifter system is a bit different. Do you have a specific tub/model? The dealer can provide you with minimum cleareances for each of the lifters the provide with each tub.
When I was looking, 15+20", depending on how you mount the lifter, seemed to be what I was hearing for min. clearence. My cover drops down about 50% behind the tub, as opposed to a higher sitting cover like the above pic, and my lifter takes about 18" of clearence.
YMMV.