They use the legal minimums for commercially-kept chickens. Commercial chickens are intensively managed and specially bred to tolerate tight confinement.
If you're in the warmer part of Tennessee you'll probably want extra ventilation beyond the minimums -- unless you can put the coop in deep...
Welcome to BYC.
@rosemarythyme has given you excellent advice so I'll focus on helping you with the math for your desired flock.
You currently have 5 birds, 3 of which are bantams, and want to add another standard-sized bird, right?
I'm going to work the math for 6 standard-sized hens both...