A 4x8 is the largest I'd build that was not a walk-in. You need to be able to reach everywhere inside so build access doors, probably from inside that shed.
I would for lighting. You should get quite a bit of light from that venting but I'd probably put one on the back wall. It won't hurt and might help on those dark dreary days of winter.
I lived in Northwest Arkansas and had chickens. I think that should be enough venting. Just make sure the wind does not blow on them when they are on the roosts.
Can your chickens fly? If they can fly, they can fly up that high, even without a ramp, if they want to. If you have some that can't fly, like Silkies, they can still be trained to use a ramp. The trick is to teach them that the coop is where they want to sleep.
It has been a while but there are some older threads on this forum where people had openings that high. I remember one that the coop was in the upstairs of a barn with chicken access through a wall.
Aart makes a good point about the dust but it sounds like that doesn't concern you with what you plan to have in there. I had a lot of ventilation on the interior wall when I closed off the end of a shed to build my coop. Depending on how drafty it actually is you might consider closing off the lower section of the coop and having the top part open.
How do you plan on providing heat to your brooder? Lighting is convenient but you don't need it. I did not heat my coop in NW Arkansas either but I did have a built-in brooder and needed electricity for that.
How high is up? How far away is far away? Yours is another question that doesn't have a fixed answer. I've seen different guidelines on this forum. The trendy guidelines right now are for 4 square feet per chicken in the coop and 10 square feet in the run. These work for the majority of people provided you don't integrate. But occasionally they are too tight. You should have weather where they can be outside all day practically every day. I don't know what you plan as far as a run goes or how much you will keep them locked in the coop only.
If you have to have a number I'd say no more than 8. But I kind of cringe when someone asks how many chickens can I shoehorn into this space. I'd prefer to look at it as how many chickens do I want and then decide how I can provide enough room for them.