Rousey was embarrassed by Holm. Floyd is stronger, faster, more accurate, and more experienced. Rousey doesn't have the stamina to handle with Holm. There's no way she can hang with Floyd. Rousey has a 0% chance against Mayweather.
Mayweather also has another huge advantage. Pound for pound, men are naturally stronger with broader shoulders, shorter legs, bigger hands, etc. In short, men are naturally better-suited for combat. I don't think there's any debate on this issue, but don't take my word for it.
Rousey herself has cited "bone structure" as a reason for not wanting to fight transgender fighters. This is what Rousey has said about the prospect of fighting transgender fighter, Fallon Fox:
"She can try hormones, chop her pecker off, but its still the same bone structure a man has. Its an advantage. I dont think its fair."
If Rousey can use "bone structure" as a reason why she won't fight transgender fighters, then I will use "bone structure" as one of the many reasons she would have no chance of beating Floyd.