Wow! To hold the weight of a person, the hub will need to be extremely heavy duty. I would search the local auto wreckers for a used wheel hub. I'd then use the rim of a spare tire to mount to the backside of an actual large wheel.
I made an oversized "prize wheel" last year, and with mine one of the key issues was balance. Balance had to be spot on, especially with a bigger and heavier wheel. You could search craigslist & Goodwill for a large round wooden dining room table. This would have a finished edge and might split into 2 parts for handling. Sounds like a bold venture- good luck and take some pictures if you pull it off.
And PS, don't forget a pickle fork and big hammer if you go scrounging at the junkyard!
First thought is that a junked boat lift wheel might have the size to start building one out. But, you will typically need to engineer and weld up a stand (and spinning hub, if you're crazy enough to actually rotate it). Sounds like mayhem (for your wallet) and madness (for your brainbox), this project. Definitely not for someone without the technical chops, especially if your putting others at risk.
How is your project coming? For a wheel that would hold a person, I would use the spindle, hub and bearing assembly off a wrecked car. Mount this to a steel frame you could bolt this together just use Loc-Tite on the nuts. For the wheel, glue and screw together 4 layers of 3/4 inch smooth plywood. Alternate the direction of the panels so that no seams from one layer line up with those of the next. Cut your circle and mount it to the hub.