Just make them bigger. Rule of twos... twice as big, twice as much ventilation, it will cost twice as much and take twice as long. BIGGER IS BETTER
Gary
2-3 sq ft per chicken is not the number I see posted often here. 4sq ft in the coop and 10 in the run is what is commonly posted as a MINIMUM
And 8x4 or 6x10 is a pretty small coop.
@aart is right. You are misleading people using 2-3 sq ft per bird.
Gary