Floyd, in his prime would probably be a junior welterweight. That would be before he started aging and before his hands got beat up and he had to morph into a defensive-minded fighter. This means Tszyu would have a huge size advantage. That being said, it wouldn't matter. Tim is powerful and has the proverbial puncher's chance but he is a slow and plodding fighter and there is a much better chance that Money would win a comfortable decision.
Floyd has already ducked Tim in the gym in Las Vegas. In his prime, he demonstrated that the best way to beat a Tszyu is to not fight him. When Tszyu was WBC champ at Light Welter, Floyd waited for the WBC to order Tszyu to defend his title against undeserving Italian mandatory, Gianluca Branco. Tszyu, who was tied to Showtime at the time, couldn't do that fight b/c Showtime refused to accept Branco as a viable opponent. The WBC then ordered Branco v Arturo Gatti. Gatti won & Floyd challenged him for a belt instead of having to face a Tszyu. True story.
Mayweather's next match was the long-anticipated fight against six-division champion and WBC light-middleweight titleholder Oscar De La Hoya on May 5, 2007. De La Hoya's belt was on the line, which required Mayweather to move up in weight from 147 pounds to 154. However, Mayweather was outweighed by more than 10 pounds the night of the fight, coming in at only 150 pounds. Fought Cotto at 154 as well yeah? Anyone else? Floyd beats anyone. Floyd's the best ever. I'm one of the biggest Tszyu fans fact. But get real. Floyd would kill him with his jab alone. Then comes the lead right hand and bob. He'd would shoulder barge, and crack. Lead left hook. Stiff left jab to the body. Floyd schools him.
Floyd only takes the fight if Tim agrees to a 152 pound catch weight with a rehydration clause of 156 pound max meanwhile Floyd is allowed to rehydrate with saline drip in his house. Kenny Bayless as the referee Tim also not allowed to wear his usual Everlast gloves.