Jones has longish arms (6'5" - comparable to like, Coby White), so his effective size is bigger than his height.
He's not someone whose game I'm super familiar with, so I concede I'm mostly just going off the various metrics, but he grades well both offensively and defensively. He hasn't been a great three point shooter in his career (.333), but is an elite free throw guy (~85% the last two season), so presumably has some more upside there.
Given his effectiveness, age, and likely cost, he's someone who makes a lot of sense to me for the Celtics.