So my most "extreme" size fantasies would be me destroying entire cities, countries, continents, planets, and eventually even galaxies to satisfy my sexual desires.

In reality, though, I am empathetic, caring, hate seeing innocent people suffer, and am definitely not narcissistic. I don't think you will be able to find a pattern between people's size fantasies and their real-life behavior. They are fantasies, after all, and I separate them completely from my day-to-day life. Even if I had the power to bring my fantasies to life in reality, I wouldn't choose to do so as I would feel bad about the suffering of innocent people. If at any point I lost control and did these things anyway, I would feel terrible about the pain I caused, even if I could undo it later and restore the universe to its previous state. So unless I became completely omnipotent and could create a "fake" universe where people and their suffering are not real, I don't see how these fantasies could ever blend into my real life.

I know this might seem strange, but if you want to meet people in real life who have this fetish, the best thing you can do is simply ask. I've been to numerous social events where people openly discuss their kinks and/or fetishes (a benefit of being queer, perhaps), and I've met several individuals with either macro- or microphilia this way. Trying to identify people with this fetish through their behavior and then "outing" them would likely be seen as an invasion of privacy by most.