With how he has been dropping battle Royal rule and saying they think he will be at least High B Low A in the first as their predictions. I would say MID A.
I would say 5' 9" or finally get to average hight but if his body catches up to device level???? hmmmm. I would still have to say an average of 5'9'. I really think it would be too hard on his tendons and legamates to grow to fast. He would rupture one or the other just keeping up with his increased muscle strength and density.
Externals would rock. I don’t think they hinted at anything but gravity assisting externals but it would be cool if they could grant abilities, and when they get destroyed he can’t do that ability anymore.
Shido seems to be open ended so I think echo but able to keep the ability it steals would be cool. It’s hard not to imagine the character being super OP too quickly, though.