Using tiresize.com and entering my tires (275/65R20 my old post was wrong and I updated it just now) and the checking their tables, it looks like 295/70R18 would be a safe bet…HOWEVER, that can vary based on brand entire inflation and other things. According to their site my tires will be 34.1 inches in diameter. But when I measure them with a tape measure they’re a little less than that. To be super safe I got my “full size” spare slightly small. It is 275/70R18. On paper that is a 33.2 inch tire, so safe as a spare for a 34.1.
Then there is back spacing differences and other things.
You might hit a tire shop and see if they will let you test fit.