RBV is definitely enough mass in addition to the slab. The conventional rule is 6 m2 mass area to 1m2 north glazed area.
The eave length is dependent on your latitude and when the cooling season starts (and the window heights)
At 1000 mm eaves and 2100mm windows (300mm below eaves), shading of the top portion will start at an altitude of 16 degrees and be total at 67 degrees. By mid spring or autumn in Braidwood there will be complete shading of the window by midday. In fact some shading will occur within 1 hour of the sun rising. Is that what you want?
Eaves too long, block out warming winter sun. Eaves too short, let in to much hot summer sun.