Here is a sketch of my ideal sauna, drawn in ideal conditions

on scratch paper in a dimly lit changing room, with a couple beers, taking a sauna with an architect and contractor friend, while at my island lake cabin:

1. Compact: 8′x12′ overall dimensions.  Most building codes don’t require a building permit for structures less than 100 sq. ft.
2. Efficient: a 6′x8′ hot room is a nice size.  Big enough to accommodate as a one person sauna to a six person sauna, yet small enough to heat quickly and efficiently.
3. Divine proportion: the golden ratio of architecture, harmony within nature, plus all your building materials come in dimensions that offer minimal cutting and waste (to burn in your sauna stove).
4. Two rooms: Critical in cold climates.  Just as grocery stores have double doors as an energy saver, your sauna should have a changing room.  It’s impossible to have an efficient sauna if the door from the hot room opens to the outside.  Close the door!
5. Dual benches: Consider pushing your internal wall a few inches, 6’4″ bench length is a magic dimension.  One person can stretch out, laying on the sauna bench or three people can sit comfortably.  Duel benches allow for good flow.  One can alter the two bench plan, and there is a good argument that having an “L” bench instead, modesty, privacy wise.
6. Versatile: A great sauna party by night can also be a nice home office escape or kids play area by day.  Storage above your hot room, and allow yourself to make a cabin style patio, that flows out from your changing room.  Turn your 8×12 sauna into a backyard escape.
7. Odds & Ends: A few tips to consider:

• Put a dimmer light in your hot room.
• 7′ internal ceiling in hot room, never higher.  Less unneeded cube mass to heat.
• Before cedar paneling your sauna/changing room, wire for sound and Ovi/Nokia mobile device technology.  Plus, I’m a fan of music in the sauna.