I want to create a world set on our Earth but with humans shrunk down as much as possible to allow a huge population.
As the population increases they genetically modify their bodies to be as small as possible while retaining human intelligence. They do not replace their bodies and minds with machines. As a result each shrunk human requires considerably less food and water, allowing the population to increase beyond what Earth could otherwise sustain.
The shrinking is limited by the need to remain mobile. I don't want the humans shrunk down to a brain in a jar. They still have arms and legs and can move around (albeit more slowly).
If they make use of the entire surface of the Earth, collecting as much solar power as possible without overheating the Earth beyond what they can survive, and maximise efficiency by not eating meat, what is the maximum population that could be sustained long term? Assume that they drive other creatures to extinction and only sustain life that they use for food.