I don’t experience body dysmorphia, with regard to Size Fantasy or otherwise. Like @blehb, Size Fantasy is for me completely separate from my IRL physical self. I’ve never felt particularly tall or short, and I’ve never been involved with particularly short or tall women. To me, the whole point is fantastic size difference, so one’s mundane size is irrelevant.
A primary reason why I’m always insisting on more depictions of male giants in Size Fantasy is that I want to imagine what male giants look like to those who find them attractive. It’s not that I find my IRL body “unsuitable” for imagining as a giant, I just don’t kid myself that there aren’t archetypes out there that people prefer.