How large?

[Auntiejessi3]

This does not answer your actual question but if I was in the shoes I think you’re in from reading your post, I would want to know that 6 week old pullets have a long time yet before they are of laying age. In my experience the flock never really accepts newcomers until they start laying. They will need spaces to hide and escape the other adults should any be inclined to be hostile. Which you could bet money at least one will.
People have varying opinions on space, and you will have to ask yourself what your goals are. Do you want a happy peaceful flock with low stress for the birds and for yourself? If you do then 10x10 for these pullets all by themselves would be pushing that. The minimum space requirement quoted often here are minimums for factory farms, many if not all are only able to stock so many birds together by buying debeaked chicks to avoid bloodbaths.
Good luck with your flock and I apologize for cluttering up your thread not answering your actual question!

Not necessarily true, my 1 yr old BO with a 14 week "new" addition

 

[Driley62]

Not necessarily true, my 1 yr old BO with a 14 week "new" addition

Agreed, a friend just put 3- 8week old ameraucanas with his 1 year old layers with zero issue. Not saying it always happens but I've heard more often times its better to wait til they're about the same size. I have another friend too that had to wait. His 3 adults bullied 6- 8 week olds(lost a couple to a fox and one just randomly died without signs of disease)

 

[aart]

so I choose to keep their quarters somewhat tight to help them retain heat naturally rather than artificially.

You don't want your coop to 'hold heat'.
They need copious ventilation all year around so the ammonia laden moist air can escape.

 

[Driley62]

And you don't think that wasn't considered in my plans? Again college background in wildlife sciences. You're grasping at straws, you can have a well ventilated coop and it still retain heat. I know because I know a few people that accomplished exactly that. To the point when it was 0 degrees outside he stepped in the coop and took his coat off. No moisture or stench.

 

[aart]

And you don't think that wasn't considered in my plans? Again college background in wildlife sciences. You're grasping at straws, you can have a well ventilated coop and it still retain heat. I know because I know a few people that accomplished exactly that. To the point when it was 0 degrees outside he stepped in the coop and took his coat off. No moisture or stench.

Sounds like you've got it all figured out.
Best of cLuck to you!