Really glad someone noted that lumber is more cost efficient in 4 and 8' lengths, I forgot to mention that. $200 is a pretty good cost savings just for changing the dimensions a little and you're only losing 2 sq ft in the end.I actually just finished pricing out materials for a 4x12 coop, rather than a 5x10. It was about $200 less for materials!
I agree I definitely want a walk-in coop. The prefab coop I have now is not walk-in and it's so difficult to clean and access.
Also agree on walk in. It's just so much easier to maintain (little to no bending down, over or kneeling) and also gives you easier access to the birds if you need to get in there to put them on the roost, nab them for health reasons, etc.