Size appears to be folks' primary concern, but Bowe's sole loss (which also happened in Bowe's prime) was to Holyfield who was basically the same size as Joe Louis. Same height, same weight. Except Joe is one of the hardest punchers in Boxing history, and Holyfield never had a Heavyweight punch. Holy was a good combination puncher, Louis was a better combination puncher...and with harder punches.
Joe was fundamentally sound, one of the best schooled, textbook fighters there's ever been, whereas Holyfield fought off-balance a helluva lot.

Bowe was damn good for a short time, and had wonderful physical attributes, and was a fantastic inside fighter considering he was so tall with those long arms. He was also psychologically unsound and lacked discipline.
6'5", 235 lbs was about his best weight, 246 or more was too heavy for him.

Joe Louis decimated big men like:
6' 4", 254 lb Abe Simon
6' 5½", 260 lb Primo Carnera
6' 4 ½" 220 lb Jorge Brescia
6' 6½", 250 lb Buddy Baer

Smaller, faster, speedier guys gave Louis more trouble to catch up with them as they skedaddled around the ring away from him. I think a prime Joe Louis would knock out a prime Riddick Bowe, but it'd be a great fight, probably go 11 rds maybe.