@TakoAlice8 First and foremost, I’m so sorry you’re having difficulties with body image. Your health is the most important thing, and as long as you’re leading a healthy lifestyle and communicating with your doctor, your shape and size are perfectly fine.
If you feel this is causing you a lot of distress in life, it might be a good idea to speak to a professional? Genuine body dysmorphia is a common thing, but it is important to address if it is having a negative effect on you.
As far as fantasy goes, I am strictly in the shrunken woman role. I get extremely uncomfortable imagining myself as the larger half in a size interaction. I know that’s very dramatic (I like to think of myself as pretty laid back normally), but for some reason this particular dynamic feels way too awkward. It’s even more strange because I enjoy good giantess content! I just can’t insert myself into the role. I have no idea why this is.
As for real life dysmorphia? I don’t really experience any when it comes to size. I’m at a very average height and ideal weight, so I don’t really feel “too big” or “too small.” Growing up I was always one of the tallest girls in class, which made me feel pretty insecure at that age. You would think that might be the source of this fascination of mine, but I was dreaming about size stuff looong before I started noticing things I didn’t like about my body. 
My younger self would be happy to know I stopped growing around middle school. Unfortunately, my adult self wishes I had longer legs. 
Which I guess brings me to my next point.
I don’t really associate size kink with real life height or weight, because the two are completely separated in my mind. Imagining myself as a giantess is very unpleasant, but I wouldn’t mind gaining a few inches to have modelesque legs. I don’t have a strong height preference for men- I’ve been into guys that towered over me, and guys that were shorter than me. When I’m coming up with stories and imagining the characters, I like to imagine them as all shapes and sizes. Rarely does it have a big influence on the plot- I just enjoy thinking of what is most fitting aesthetically.
The size I want to be in my fantasies is completely divorced from reality, so no realistic size is going to do anything for me.
Sorry for writing so much, but I think this is a very interesting topic! Thank you for posting! I hope my response was a little bit insightful. 
But if you are ever feeling down about this sort of thing, feel free to reach out. Unfortunately, there’s not a whole lot people can say to make someone with body dysmorphia feel better. But sometimes it is nice to just talk and get things off your mind. 
It’s so important to respect people’s boundaries!