The sauna bench is the most underrated part of a cabin. Heat rises, so bench height decides what temperature you actually experience, and bench depth decides whether you can lie down. A beautifully built room with badly placed benches is an uncomfortable sauna, and it cannot be fixed with the heater.
Height against the stove
The upper bench wants to sit level with or above the top of the heater stones, because that is where the steam goes. Below it you get the heat but not the loyly. The gap from the upper bench to the ceiling then matters more than the floor to ceiling height: keep it modest so the hot layer is around you rather than above you.
Depth and length
A bench you can only sit upright on is a different room from one you can lie on. Lying needs real depth, and a length that suits the tallest person using it. Where the plan will not allow it, an L shape can give one long run for lying and a shorter one for sitting, which is the usual compromise in a three or four seat cabin.
Timber and the gaps
Bench timber is the wood you touch for twenty minutes at temperature, so it should be low density and knot free: aspen, alder, western red cedar and thermally modified softwoods are the usual choices. Leave gaps between the boards for drainage and airflow, and fix from below so there is no metal near skin.
Making them liftable
Benches that lift out are worth insisting on. It lets the floor under them dry and be cleaned, which is what keeps a cabin smelling right after a few years, and it makes sanding the boards a job you do outside on a bench rather than crouched in a hot room. Few makers offer it by default and most will do it if asked.
Questions people ask about sauna bench
How high should the top bench be?
Level with or above the top of the heater stones, with a modest gap to the ceiling so the hot layer is around you.
Which wood is best for benches?
Low density, knot free timber: aspen, alder, western red cedar or a thermally modified softwood. It is the wood you sit on, so it must stay cool.
Should benches be fixed or removable?
Removable if you can. The floor underneath dries and cleans properly, and sanding the boards becomes a job you do outside.