A full-size spare will fit. Your will also need to purchase tools to put on the spare. The OEM hitch for the MDX can also fit a full size spare if you are thinking of adding one down the road. Some aftermarket hitches that hang below the bumper can't take a full size spare.
I purchased the spare tire kit minus the donut spare at time of purchase for my 19 MDX hybrid. I purchased an used black 20" Berlina rim and used tire from eBay for about $250. I just added an OEM hitch/harness last March before the lockdown for a bike rack.