I'm building a 32' diameter Yurt at the moment, and I plan to put a rocket stove mass heater of 8 each in the middle of the Yurt. I did a plan, and the total height of the stove (with the barrel) would be close to 60 inch (maybe a bit more). I'm planning to put a mezzanine in the middle of the Yurt. The mezzanine floor will be at 7 feet height.
So I will get something like 24" beetween mezzanine floor and barrel of the stove. In the center of the mezzanine, I will make a trap to be able to fix the barrel on the stove etc (48" diameter). Did 24" will be enougth of cleareance ? Maybe I will need to put some insulation under the mezzanine floor ? in the middle of it. What did you think ? the mezzanine will be make of tamarack (larix laricina).
32 feet diameter in a round Yurt give more than 800 sq feet, I got like 100 feet of wall. Im near Québec city, 500 m elevation approx. In winter I can get some nigth at -30, -35 degree. The isolation of Yurt is near nothing, it's just a double thermofoil 1 sided aluminium.
The ductwork have to be think again for sure, but the vertical chimney exit is something to forgot I guess, I get a plexiglass ligth shaft of 5 feet in the middle, the compact version with mass in the middle is cool, but I need to put the chimney exit somewhere... if I put it just after the plexiglas lifgth shaft, at 5 feet of the middle of the Yurt, unless I will have to cut some of the trust. I can put the stove near the wall... but will loose some of the heat for sure...
Since you already have your floor built, you will not be running the duct embedded in subfloor mass as some have done in yurts. You very much need to decide how you want your interior layout to work so you can plan your duct routing. You will get more reliable operation if you can run the chimney up somewhere inside the yurt instead of going outside at floor level. At or next to your central skylight would be ideal.
An 8" system can handle up to 50' of duct (minus 5' per 90 degree elbow), so with a minimum of two elbows, you could have a 20' long bench. More elbows would mean a shorter bench.
You might also want to try a system known as "half-barrel bells", where the duct is replaced by barrels cut in half lengthwise and laid on their sides. This allows better heat transfer with less friction, and would also allow benches to be lighter for the sake of your floor loading.
The barrel in a standard RMH is much like a woodstove in temperature, and needs the same clearances. Code is around 48" to unprotected combustibles, and more (60"?) to overhead combustibles. Even fudging on code strictness, I would not have wood two feet over the barrel top. I think you need to leave the access hole you show open for safety. The chimney can go up in there, too. It will be cool enough that you can be safe with smaller clearances - a RMH chimney will never be 300F like a woodstove chimney is supposed to be.