Between 220 and 230 pounds. Holyfield fought Tyson at 222 lbs and he was formidable. He faced Bowe in all the 3 fights under 220 lbs achieving good results but not excellent.
About 215-220 IMHO - he's never been a power puncher, so for me he'd be better served staying at a more natural weight and playing to his strengths (speed and stamina) than bulking up for extra power and potentially diminishing his strengths.
He’s needs to maintain a high work rate and good movement to beat Joshua. Coming in a 230lbs+ butterball will just make him an easy target for that counter right straight.
This. 215 seems about the perfect weight for him at heavyweight. If Usyk starts putting on extra size it’s going to have a negative impact his mobility and handspeed.
Personally, I would like to see Usyk come in and fight his fight, and not worry about whether the judges will actually give him a win on the cards. Losing in a blatant robbery would be better for him than going in heavy in an all-or-nothing attempt to get a KO and getting stopped himself. Haye was relatively successful at 210lbs even managing to run around on a broken toe. An extra 5 pounds hasn't caused Usyk too many issues for his movement of late. Plus, Joshua isn't exactly effective at tying up, so he doesn't need to worry about fighting him in the clinch. Stick to around 215. I don't see the benefit in putting on yet more weight.