Really good question and tough one to answer. Especially if you account for evolution. Would a micro-version of human ancestors have developed all the traits that have enabled our technological progress? Maybe not, but who knows. But if we stipulate the same mental capacities, I think humans could be quite small and still be at the top of the food chain. Consider the giant machines, weapons, structures humans build today. We use machines to build machines, and other machines to lift and operate those machines. That dynamic would apply to gopher-sized people too. They could build a tank, tranquilizer gun, a skyscraper, a nuke. The bigger the size differential the harder everything gets, but there's no strict minimum limit.