Who was under the rule of the Rome Empire when it hit its largest size?

Question

Was it Hadrian?

Answer 1

I think you meant to ask "Who RULED the Roman Empire when it hit its largest size?"

 

That was Trajan.  The empire reached its greatest extent under him.  After Trajan came Hadrian, who decided the empire was too vast to defend, and actually started to pull back at some places.