Well, I have not really done a study of this yet. I might just start one next week.
But, there are some problems: the data is poluted and anecdotal. In the case of American fighters, we have the problem that the figures for height and reach are varying from fight to fight. That is becaue of reporting values. There are very few official scientifically conducted measurements. Example: Vitali is given in Europe as having a reach of 198cm and other times of 200cm. That is 78 inch and 78.75 inch respectively. But Americans report him as 80 or 81. Which figure to use?
Ali had reported values between 79 and 83. Obviously his hands were not growing or shrinking in length between fights, All these fugures were plucked out of the air with no valid ground for it.
Wlad has 200cm or 78.75 based on his gym and trainers in Europe, but Americans report him as 81.
whom do you trust?
Tua, I saw him reported from 69 to 70.5 inch. Grant and McCline from 82 to 86. Which figures to use?
Same for heights, but not so much difference: Toney was given as 5-11, now as 5-9. Tyson as 5-10 and 5-11.5, Tua as 5-9 to 5-10, Lewis as 6-4 to 6-8, Vitali as 6-8 or 6-5, Wlad the same. Byrd as 6-0.5 to 6-2.5, Valuev as 6-11 to 7-2, etc
There is more, but you get the idea.
So I could easily make a study, but sicne the values as not really valid, there cannot be much confidence in the results.
I can just pick the values of boxrec, but they got them also from people who didnt really measure them scientifically. So there is the difficulty.
Also, the height and reach advantage has to be considered also in terms of ranking. I cna bring a basketball player with 2 amator and 3 pro fights, and with height 7-4 and reach 98 and stick him against Tua and he will still lose badly. They people would draw the wrong conclusion, saying that reach and height dont matter.
Or I can point out to Lewis v Tua and show how important they are and how effortlessly you can win if you do have large advantage in this areas.
the anecdotal evidence is that for equally gifted fighter, a large reach and height advantage gives the winning edge. Only with lot of difficulty would a smaller man win. It can still happen.
A case in point: look how hard Tyson had to fight against Razor Ruddock to win, and how easily Lewis destroyed him. Or look at Tuamans fights with Lewis and then look at the Lewis Rahman 2. Lewis reach allowed him to get to Ruddocks and Rahmans jaws far easier that Tyson and Tua. And surprise surprise, Tyson and Tua are far smaller than Lewis.
Is there any doubt that if Tyson or Tua had 5-7 inch reach and 4-5 inch height more than they have they would have been a lot better, if they also kept their power and chin?
Would a 5-10 Lewis have won the main fights of his career? Would a 6-4 Tua have lost to Lewis?
So yes, other things being equal, having more inches will help a lot.