Don't know where you are located so I don't know what kind of weather you are facing. Don't know how much room you will have outside or when that outside space would be available. Both of those are big issues.
If you are buying building materials most items come in 4' or 8' dimensions, at least the less expensive common versions of those materials. If you keep those dimensions in mind you can usually save cutting and waste. 6' isn't that bad, the cut-offs can often be used to build nests.
Will it be elevated or on the ground? If it is on the ground and not elevated I'd probably want a walk-in.
Without more information I don't know what to recommend. A 4x6 would probably make a real nice coop in most circumstances.