The usual guidelines are:
For each adult, standard-sized hen you need:
4 square feet in the coop,
10 square feet in the run,
1 linear foot of roost,
1/4 of a nest box,
And 1 square foot of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
So for 8 chickens you should need:
- 32 square feet in the coop. 4'x8' is approaching the limits for a non-walk-in coop even with the access door in the middle. 6'x6' should be walk-in because even the tallest chicken-keeper won't be able to reach the far wall.
- 8 feet of roost
- 80 square feet in the run. 8'x10' is a nice looking number but, remembering the common dimensions of lumber, a roomier 8'x12' is actually easier to build. 6'x14' looks good on paper but would require a lot of weird cuts.
- 8 square feet of ventilation.
- 2-3 nest boxes.
@Ridgerunner has an excellent explanation of why these guidelines are GUIDELINES, not hard and fast rules:
https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/
And you can find a discussion of when you are likely to want more room than these numbers say:
https://www.backyardchickens.com/th...ave-more-than-the-minimums-for-space.1462570/
Since you're in northern Maine you will want to consider that severe weather that cuts off chickens' use of their run is one of those situations when the flock is likely to benefit from extra room. There is an excellent article on cold weather chicken housing here:
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
My Little Monitor Coop is designed to meet all the minimums for 4 hens and the design could easily be expanded.
https://www.backyardchickens.com/articles/the-little-monitor-coop.76275/
Good luck with your project!