Plastic Man is one of those heroes that the goofiness of his powers really betrays how strong he is. Along with being able to stretch his limbs, Plastic Man can manipulate his size and his density.
He can shape his body to into just about anything he sees, including something as large as a t-rex or as small as a bottle cap. We've never seen Plastic Man really have a limit when it comes to how large or how small he can make his body grow.
Given how elastic his body is, taking a punch from some of DC's heaviest hitters wouldn't really phase him either. Since he can also increase his density he's capable of delivering some haymakers of his own.
We saw in Injustice that he's also capable of subduing someone like the Flash by snaking his way into someone else's body and tightening his grip on their wind pipe.
The only way that probably could stop Plastic Man would be to steal an idea from Dr. Doom over at Marvel and simultaneously stretch and freeze Plastic Man until he shatters. Otherwise Plastic Man is unstoppable.