If you really do want to keep 50 birds , then you may have to extend the length some. Other option is to make it quite high to increase the interior volume. .. Needless to speculate at this stage since you are still a way from your finished product. Are you planning to have the floor like a grate so all the chicken gumballs hit the ground underneath?? Then move to new area after ground becomes fertilized ? Will your chickens be confined to coop only, or will there be a run besides??? If you move coop too far from area, your chickens may be getting lost if they free range.. I know this from some bee keeper friends and their bees. Obviously if you move it just a few feet, and in same open area. chickens will find home.. I know you are making your project from what you have. I am with you there all the way.. I myself like to repurpose anything I get my hands on. This is just a question or Idea.... have you considered converting an older travel trailer camper into a coop ??? It has wheels and walls and windows and door. All need be done is gut the interior and make into chicken housing.. In my area, these units are available if you look for them.. They are old and so are not worth very much cash. Many can be had for just removing them. Some of the more healthier models can be had for a reasonable price. You have a coop in no time..
WISHING YOU BEST.. I will watch as your progress moves forth.
Don't forget to ask for ideas and advice as project unfolds.