RMH pebble style - input please

After a few years :/ we are starting the RMH build.

Climate: Mediterranean climate ; 3 months of wet winters, 3 months of 86+/35 degree heat. Humid climate.

Area to heat: single concrete brick room(tiny house) of 35 meters square.  

Floor solid solid solid rock concrete floor

We are working off pebble mass design.
Don't, cant or won't make cob.  Could say we don't have clay in our soil...

Question 1 based on Pebble RMH design:
Instead of a single bench could we do a L shape of following dimensions?
The arms of the L are 2.6 meter( 8.5 feet) long & 1 4 meters (4.6 feet) long.  Width 0.6meters (2 feet)

Question 2:   8" duct
If we cant find 8" duct in Portugal can we do entire system in 6"?

Question 3
Can we exhaust out the chimney on the opposite end of the barrel side?

Question 4 - mass
Pea gravel- could you explain what size this is?  We work in another language here being Portuguese.
And could we use the old red clay bricks as part of the mass and infill spaces left with pebbles?

Pebble or gravel better for holding heat?

Here pea gravel is around 3/8" and I usually buy bulk so 20% or so it to big or to small. In metric that mean screen it and any thing bigger than 10mm save for something else and any thing smaller than say 6 or 7 mm screen out also every thing in between should be fine. Also you can use some large rocks in the layers for mass but because you still want airflow I am not to keen on the idea of the bricks except for maybe a top on it and then you will need air holes on the side. As for your other questions I will leave that to the people who know this far better than myself Good luck with your build.