Not just enough room for deep litter but enough room in there for you to be able to work and have some extra room for flexibility if something happens.With my current coop as the point of reference, a walk in coop with enough space for deep litter is a must for sure, so I'm designing with that in mind.
I made a couple of my nests so I could lock a hen in there if I needed to. That's come in handy for several different things.
I have electricity in the coop so I built in a 3' x 6' brooder under my droppings board. It's elevated and has a 1/2" hardware floor. The droppings fall straight through and are easy to clean, plus it keeps the brooder very dry. If I need a solid floor, say it is kind of cold, I put in a piece of plywood to cover the wire. With the wire floor it serves as a broody buster when it is not in use as a brooder. I can use it to isolate a chicken if I need to. With deep litter you probably don't need a droppings board but something to help manage the chickens can be really helpful.
I integrate a lot of chicks. I put a juvenile roost, lower than the main roosts, horizontally separated from the main roosts, and higher than the nests so the juveniles have a place to sleep that is not my nests.
My main suggestion, build it for your comfort and convenience, don't worry too much about the chickens. If you are comfortable in there they should do well.