I think the 20 x 20 is about as minimum as they can logically get, assuming there is the 16' wide roll-up or two singles of min. 8' width. The capacity of the garage is, IMHO, very dependant on the number of spaces directly accessible at a drive-in point of access. Larger structure dimension can be great for storage of their STUFF, and everybody has stuff, but the # of "cars" that you assign the amenity are determined by the drive-in points of access.
The "tandem" garages define this well, for they have extra depth, but they ultimately create only that opportunity to say the garage is "over-sized". One can comfortably have their work-bench or shop area in the back, but day-to-day use for jockeying 3 cars in and out is not reasonable. Every now and then you will find some guy who has his old Corvette or MG Midget under a canvas cover and "stored" in that forward space, but......it's still storage. If the subject has "tandem" then seek comps with the same.
Very common today to find the stand-alone 3rd space having a roll-up with at least 1 ft. more clearance to compensate the new SUV heights. Note that height aspect too, and then wonder how it is that some who buy older homes end up having to leave their monster vehicles parked outside all the time.....garage not deep enough and doorway not high enough. Oooops, we forgot that factor when we looked at the home before we bought it. Fell in love with the updated kitchen but never thought much about the garage.