AJ will stop him surely. The size difference...
I could see that being the case, but it's worth remembering that the guys AJ has looked least impressive against are probably Takan, Parker and Povetkin (until the stoppage). Povetkin is 6ft 2" with a 75" reach. Parker is 6ft 4" with a 76" reach. Takam is 6ft 1.5" with an 80.5" reach.
Usyk is 6ft 3" with a 78" reach, so he is taller with a longer reach than Povetkin, only 1" shorter than Parker with a 2" longer reach, and he's taller than Takam with a 2.5" shorter reach.
It's not the gulf in size people are imagining between Usyk and many of the other competitive heavies. He will make AJ miss and look limited, but he has a big disadvantage in reach that he will need to negate. Someone like Povetkin was pressing and jumping in to land hooks with real heavy level power. Usyk likes to box and is not a banger...that's harder to do with a big reach disadvantage.
It's a fascinating fight though. Usyk can win it, but he better have a very good chin, because at some point he is going to get tagged and if AJ is struggling he will probably throw caution to the wind and try to rush him with high volume and pressure. Usyk is not slipping everything and despite his skills he can sometimes be a bit upright too. He slips more through judgement of distance and footwork than he does upper body movement, and it was the latter which AJ found difficult to negate with some of the smaller guys.