Havyweights Primo Canerra was 6'6" 250 and although most of his bouts were controlled by the mob he did know how to use his height advantage especially because he punched with very little power!
Ali might have had a tuffer time if he was 6' tall. His reactions and eye hand coordination was above everyone elses but his unconventional habit of "leaning back" from punches wouldn't be as effective without his 6'3" advantage!
Wlad Klitz has used his height advantage as well as anyone through boxing history! His height gives him a reach advantage also and without his 6'6"/7" height I don't believe he has the success he's had! His skills are ordinary and his power is above average but his height is the deciding factor against inupt challengers who don't have the skills or willingness to get under him!
Tommy Hearns obviously used his 6'1" at 147 to a huge advantage to go along with dropping a huge right hand! Tommy used his height well but was at a disadvantage once his opponent broke inside him! Also those long skinny legs weren't the best "shock absorbers" when punches bounced off his chin! Hearns had more heart than most, thats for sure!
Jess Willard 6'7" thats about it for Big Jess he was a tuff SOB but if he was 6' he'd never have the success he had!
Sndy Sadler at 5'9" 126lb featherking had a huge advantage in size but he didn't need it hahaha!!
After being stopped in his second pro fought he fought 161 more fights and was NEVER stopped again! He was a big feather that could punch holes in walls!
Theres plenty more but heres a few I think are pretty meaningfull. Ray