If you were building a chair for someone 6' 10" and 250 pounds, how would you adapt the plans for the group build chair?

Maybe scale it up 20%? What about strength? Normal chairs don't hold up in service for him.

With the caveat I haven’t seen the plans or design of the project.

really just adding a bit of height to the top of the seat and maybe a bit of front to back depth. Maybe an inch.  Grab some 2x stick and drywall screw it together to experiment. It also somewhat depends on build, if you have a taller torso like me or have longer legs. 

Assuming good stock selection and techniques, you probably don’t need to make it more strong. I’m over 250 and have no issues with chairs holding my weight. (Assuming they are not loose) If you’re really worried about it, make the tenons a bit wider. For example instead of 5/16 you go up to 3/8.