Depends on a large number of factors like with or without gloves, how trained you consider trained to be, how much bigger the other person is, drunk or sober etc etc.

I guess size is not always a true indicator of the power punch someone can take if they're not defending against it in any way.

I think skill makes up for a lot but whether or not a person can be KOed depends on too many factors. Sometimes yes, other times No.