There is no immediate need for humans to become taller or shorter in the remote future. If mass famine around the world were to happen next year, which would result in poorer nutrition, then you would probably begin to see a drop in regards to human height. Mass famine is unlikely though with the only threat coming from wheat leaf rust which spreads like wild fire and has the potential to deplete the worlds wheat sources.
Fire arms play absolutely no part in human evolution and as for being as small as ants... that's just ridiculous.