At 135 or 140. In this match up we have one of if not the best at imposing his will Benny Leonard against One of the best if not the best at adapting in the ring Floyd Mayweather Jr. I think this fight can take place at lightweight or light welterweight. Mayweather is a former champion and Leonard fought sevrael times within the 136-140 range. Mayweather's size could be a factor 5′ 8/72" reach compared to Leonard 5′ 5″/69" reach. My opinion is that Leonard being able to dictate how the fight goes gives him the advantage. Mayweather will adapt, but Leonard will keep control of the fight. Leonard forces Mayweather to lead and is able to make up the size difrance through his counter punching. Mayweather will adapt and get more comfortable with his role in the fight and win rounds of his own. However with Leonard winning 5/6 rounds. Mayweather will find him self in to big of a hole. The next six rounds are split 3-3. Giving Leonard at least a 8-4 win or lead if schuled for 15. Meaning that he would do do worse than win 8-7 over 15. At 135 or 140 the fight is the same in my opinion. It is my opinion that Leonard and Pernell Whitaker are the only boxers who are smaller than Mayweather that I'd pick to beat him. I am more confandt that Leonard beats Mayweather than I am about Whitaker.
Mayweather is loaded with talent, but Leonard has no real weaknesses. He would own Floyd by a UD that isn't even close.
This would be a close fight without much between the two men. Benny just has that little bit more to offer ad i think this would be the case in most rounds with Benny taking a wide UD.
This is one of the relatively rare instances where their eras would be important. Many, many times far too much is made of "modern training methods, nutrition, blah blah blah", but when it's an extreme case like this of a guy who peaked in the 1920s versus one of the best athletes of the 2000s, I strongly favour the athlete of the 2000s. In terms of career greatness, Leonard is in a different dimension to Mayweather of course, but put them both in a ring and I think Benny would find Mayweather's handspeed and slickness far too much to deal with. I vehemently disagree when I see people pulling out the old "modern training methods, nutrition" dog**** when it's an h2h encounter between say a 1970s fighter and a 2000s fighter (Napoles > Mayweather IMO), but when it's a 1920s fighter I think this argument has some merit.
I hoestly don't see this ending before the final bell. Both are pretty resellant and have great defense. Both of their power is underatted, but I think the above factors means they go the distance.
Mayweather is bigger and stronger and that can't be ignored, as it could be a deciding factor. However there is going to be some debate as to who is faster. Most people would say Leonard is the better boxer.
Personaly outside of the heavyweights I'm not sold on the modern training argument. 135/140 is 135/140 no matter the eara. I think the more modern boxers do have a strigical advantage as they can learn from the past and see what works and dose not work in the ring. To this end Mayweather has the advantage. However both boxers are going to be in shape and ready for 12/15 rounds.