Mike Tyson. Anyone before really the 90's would be considered small today, a lot of the top guys I think. The 90's started the era of the Super Heavyweight.
I would want to have a build like Ali's. He was 6 ft 3 and weight about 210lbs, I'm estimated to stop growing at 6 ft 2 and will probably aim for around 220lbs, I don't think I could fight Ali style as the guys are a lot taller in the heavyweight division nowadays.
Not being too muscly, but have good muscle tone, around 6' 1", naturally a mesomorph, mentally strong.
6'1 is way too short man. Even though the mind of the boxer is the most important aspect of a sucessful boxer, but 6'1 would not be ideal. So I would say 6'3-6'5 (really 6'3-6'5 barefoot not 6'2 like 5'11 andy ruiz) and a very athletic body not too heavy not too small, with broad shoulders, long arms and powerful legs. As for weigth I'd say between 225-250lbs seems to be the best depending on build.
Tyson had an amazing physique but he was like 5'10" with a 71" inch reach. Far from an ideal Heavyweight build.