Thank you all for the nice comments. Here is how it went... I grew up with an electric sauna, and my wife and I always intended to build one once we had some acreage. I began the project in 2014, but didn't want to spend the money for an electric heater right away (I'm quite cheap). Inspired by my wife finding me an arc welder at a garage sale, I began to look for a way to build a simple wood heater as purely a temporary step until I could find a good deal on an electric unit. Right about that time I happened on a link to a RMH article and the gears began turning. Fast forward through lots of reading and many prototypes and design changes... here we are.
It honestly works far better than I believed possible. To answer Tel specifically: The overall building is 16 x 8 outside dimensions. The steam room is 8ft 5in x 7ft 3in inside with a 7ft 3in ceiling, so large for what I am used to.
I have had it up to 210F, but the stove body kicks off a lot of radiant heat - making the walls and benches uncomfortably hot to the touch. We have found that 160-180F feels really nice. What I am really happy with is the heating time. The other night it was 160+ about 40 mins after lighting the fire. That was with a load of alder and oak. I have been burning lots of random wood (pallet pieces, cottonwood, unseasoned branches), so around an hour is pretty common.
Cost: We are still tracking some things, but it will honestly top $6,000 before it's done. That was with sourcing as many discount materials as possible and doing all labor myself. There are certainly places I could have shaved costs, but I found out simply building a watertight outdoor insulated buildng is expensive.
Plans: No plans, sorry. I came up with the overall dimensions and just went with it. I will try to answer any specific layout/dimension/construction questions though.
More pictures as I dig them up!