According to the usual guidelines, 12 hens should have:
- 48 square feet in the coop. 6'x8' is more practical than 4'x12' since a long, skinny coop like that would be difficult to work inside.
- 12 feet of roost
- 120 square feet in the run. 10'x12' or 8'x15' -- 8'x16' means fewer odd cuts than either of those. 6'x20' is possible, especially if your run is an open-topped, fenced area instead of fully-enclosed with a solid and/or wire roof but risks social problems because subordinate hens need to be able to pass the dominant hens at a respectful distance.
- 12 square feet of ventilation.
- 3 nest boxes.
So 8x8 is good and 2 roosts would be good -- though as noted above they shouldn't be on top of the nests and diagonal roosts suit the human sense of aesthetics better than they suit chickens' needs.
I suggest that you raise your ventilation to the very peak of the roof -- heat and ammonia both rise -- and that you extend your roof overhangs generously to shelter your vents from blowing rain and snow and to give you a protected place to stand when gathering eggs.
Good job on the front drop down for the nests -- so much less likely to leak!
I suggest a larger access door so that, if necessary, you can get into the coop. It's never wise to have any area in your chicken-keeping facility that you can't reach in far enough to touch every square inch (my 4x4 coop I have to get my 6-foot son to do that for me).