I have no idea why not contact the maker of the sauna for all the specs . I assume this is inside so why not just get a prefab one .

this is a guide

The typical traditional sauna is 6 feet by 5 feet, with upper and lower benches, easily enough room for a family of four.Traditional saunas tend to be larger than infrared sauna sizes which require sitting in close proximity to emitters. As a general rule of thumb, traditional saunas tend to be more social, and accommodate bigger groups.

For 6 people that is actually small. In Finland and Scandinavia a 6 person sauna will typically be 2.5m x 3m x 2.6m. You want a minimum of +2m³ per person and ideally 4m³ per person. The foot bench should always be above the top of the stones thus the minimum 2.6m height.

Good info here: http://localmile.org/trumpkins-notes-on-building-a-sauna/