While humans have in the past deliberately bred animals (think "toy" dogs and cattle) to be small, so have humans incidentally "selected" for smaller animals through their eating habits. It is a well known and still contested theory that our ancestors wiped out the mammoth megafauna during the end of the last ice age, just prior to the Neolithic period, but it is less well known that humans though "over-fishing" and sports hunting are modifying gene pools where the smalle rand younger survive.
Earlier this year, Wired magazine featured an article on the phenomenon by which human hunters "genetically shrink their prey" and the potential consequences of such shrinking are.
Size is really, really important...If there is one [species] that has changed dramatically in size, its relationship to natural predators could be lost...As a consequence of targeting large adults, we're targeting reproductive age adults, so those that [reproduce younger] have an evolutionary advantage... The problem is that these younger mothers don't produce as many babies, which could have major impacts on how humans estimate fishery population and potential... [Finally], even if you could stop hunters and fishermen from pressuring the population, genetically altered animals would have to re-evolve their previous phenotypes, or forms.
Just recently, scientists have confirmed that not only human hunting has been influencing the size of animals, but human climate change as well. As has long been observed (at least in mammals), there is a negative correlation between size and temperature. US News reports that
As Earth's climate continues to warm, life might become the province of the small... Two such ecological changes [resulting from climate change] that have been noted and predicted are  the shift of species' ranges to higher altitudes and latitudes to keep within their temperature comfort zones and  the shift in the timing of key events in the life cycle of organisms... A third change can now be added to that list:  As temperatures rise, organisms get smaller, from the , from the scale of whole communities down to the individual.