Forget about Joshua's size, Opetaia has more than enough Power to knock him out brutally, I'm sure of that...
Opetaia is a good raw talent. Don't rush him. He can build his skills bit by bit. One of the things Usyk has is he has seen and faught against a lot of different styles. Bring along Opetaia carefully. He has time. Get him in with trainers and sparring partners that can teach him and challenge him. That may not be in Australia.