These guys both can alter their size at will. But the difference is that their limits are way different. The Atom can alter his size almost infinitely while ant-man is limited to insect size. This, I guess is supplemented by the fact that he can communicate with insects. So this battle wold still take place in my arena but probably on a micro scale. Their shrinking ability would be cancelled out at a certain size since relative to each other their size did not change. They seem to have equal fighting skill so that won't be a factor. Considering their other abilities, Ant-man can call upon insects to fight Atom. This would do no good since he would just shrink further, thus avoiding the insects. Atom's power centered about the control of his density and mass, therefore he can make himself really light in order to jump really far. He can probably make himself really heavy and hit hard or something but I have never seen him do that. I think that the Atom could and would go into Ant-man's head and either damage his brain to knock him out or kill him or grow big again and blow his head up.
