He can shrink to extremely small sizes, the size of an atom, but retains the full force of the Batman. Unlike the Atom, Batman has reinforced his suit within armor giving him an upper hand.
I made this Batman TAS figure, the only one I left from when I was a kid into an Atom type Batman so I could display him with my 6 inch figures. Why's he smaller, well he's got a Dwarf Star belt of course.