I read an article at Steelers depot where Mark Kaboly commented on Jones and his size. He said he looks kind of small compared to Jason Worlids and Lamar Woodley.
He does look that way but at 245 that should be big enough to play that position, although the article said he may struggle against the run getting off blocks on the edge.
I don't know if his size really matters, if he can do what the Steelers expect of him. He has the speed, now its a matter of technique and learning the defense.
James Harrison was considered small too in height but he made up for it getting an edge in leverage, however he was a workout freak who had the power to beat his man getting to the QB, and one of the best in stopping the run.
Btw, If he's considered on the lean side maybe beside getting to the QB he'll do well in pass coverage something we could use from the OLB position.