How many birds?
What sorts of materials?
Where are you located?
Are you planning to raise em year round?
Have you raised meatbirds before? If it turns out to be a greater PITA than anticipated, it'd be nice to have left some room for laying boxes in a suitably convenient location.
Are you feeding outside? If so I'd leave a generous overhang in the front to shelter the feed from precip.
My outbuildings are all more 'squarish' to maximize perimeter to volume ratio. You've got 40' of perimeter...which'd make a 10' square building, totalling 100 ft2...a 33% increase in volume for the same amount of wall.
I also build in 4' increments for ease of install (16 or 24" studs, paneling comes in 4 x 8' sheets, roofing materials similar...). Framing odd increments leads to more cuts and more waste: can get expensive in both time and materials. For a 5 x 15 structure sheathed in 4x8' panels you'd use a total of 8 full panels, plus two additional panels cut into 3' and 1' strips, 10 panels total (hope you have a tablesaw!). That extra 1 ft strip would likely require an additional stud. For comparison, a 8 x 12 structure would require no cuts, contain 96 ft2 (~30% volume increase over 75 ft2)....while also requiring 10 panels.
I'm typing from experience. My own coop was built completely backwards, from the roof down! I'd acquired an old 7'11" x 7'8" garage door, quite the robust one-piece ceiling solution. It all seemed so easy until I began sketching and testing different angles to maximize my dimensions. I'm quite happy with the results: but if I'd just spent ~$80 on roof panels + sheetmetal screws, I could've saved a ton of time and headache by just building an 8x8' building.
You could alleviate many of these issues by using different materials, but this of course leads to other complications.
Whatever you do, keep the roosts low! Meatbirds aren't so great at hopping about!