X2 - while the plan *as is* may not be large enough run wise, that is an easy fix/adjustment as it looks like this setup would lend itself VERY easily to expansion. Heck the shape of the structure would even lend itself easily to adding a "hoop coop" style run off the back of the back of the building for a quick easy "add-on"If you like the aesthetics of that structure for your coop, I say go for it. Adding more run length (either to one side, both sides, or the back) seems fairly straight forward and customizable. If space is a concern, why not start with just half of your flock in there and see how it goes, then expand your flock accordingly.
Plus, your son and husband building it from scratch, what an awesome bonding experience for them. At the end of the day they can stand back and admire what they built from the ground up. That's priceless in my opinion.