Elevating a small coop allows for more efficient use of space when you don't have much space to use. It can be very convenient to clean out when you can park a wheelbarrow or garden cart under the door and reach every square inch with your pitchfork.
Larger coops are a problem elevated because you can't reach to clean, to retrieve a misplaced egg, to get to a stubborn chicken, etc. It is possible to elevate a walk-in coop -- where I live it's normal to set sheds up on blocks to deter termites -- but you have that same "can't reach it" problem underneath.
IMO, 4x8 is the maximum for an elevated coop and that's only with a wide access door in the center of the long side.
Our coop right now is about 4ftx5ft. I’m thinking about a 6 ft length x 5 ft width.\
Lumber comes in multiples of 4 feet so that 5 foot dimension means cutting off and throwing out 3 feet of every 8-foot 2x4 you buy. Lumber prices are still high so that's a lot of $$$ in the scrap pile.
Better to build either 4x8 or 6x6 (buy 12-foot boards and cut them in half).
You mentioned having 1 sq ft of ventilation for each bird. I’m not really sure how to figure that out. Right now i have three hens and 2 two week old pullets. How many/how big should the windows be?
Soffit and ridge or soffit and gable ventilation is better than windows. Heat and ammonia both rise and need to be vented at the highest point of the coop. Additionally, you still need that square foot of ventilation per bird in the winter so having the ventilation above the birds' heads while they're sitting on the roost prevents drafts.
This photo is from an article on cattle barns, but the principles are the same:
@aart has good photos of how to make soffit ventilation.
Alternately, with generous roof overhangs you can put vents at the top of the walls by just not carrying the siding all the way to the top and using hardware cloth instead -- like my brooder, but less redneck.
If you do use windows -- and you will probably want supplemental ventilation in the summer -- best to make them top-hinged so that they serve as their own awnings to keep the weather out.
Here's a link showing how to make a safe prop for them:
https://www.backyardchickens.com/articles/window-support-for-top-hinged-windows.74810/
Is 6 inches off the ground for a nesting box too much for chickens or should i make it level with the floor?
Think about this from your perspective. The chickens don't care what height the box is as long as they can get to it. Do YOU want to have to bend over and collect eggs from the height of your ankles every day?
I have arthritis in my back and knee. The nestboxes in my
new coop are at my elbow height. I'll give the chickens a ramp to go up.
