It’s always nice to have a bench long enough for someone to lie down (6' or 7').
We can do a sauna as small as 4'x4 or 3'x5'
4’x6’ is a nice size small sauna. (Even in small saunas, we try to always fit two tier benches.)
5’x7’ with top L bench is the most popular size.
6’x8’ is recommended max. size for home sauna.