My ten-year-old and I were in the pool and he asked if I was sensitive about my size. I told him, no not particularly, and went on to clarify...
Me: If I say my hair is brown, nobody is upset about that, right? And if someone else says to me, "Your hair is brown!", that isn't an insult, right?
10: No!
Me: But what about if they say, "Ewwwwww, your hair is brown!" Is that insulting?
10: Yeah, that sounds mean.
Me: So I think the same way about saying I'm fat. It isn't a bad thing, so it literally doesn't matter.
10: But you aren't fat, you're, like, muscley.
Me (Doesn't want to get into the weeds about it): But I have a bigger body. But it literally doesn't matter. My size isn't good or bad.
We went on to talk a little about his body, and also how people will sometimes say mean things even to smaller bodied people about being "too skinny" or other things. We agreed that bodies are all different, and that's cool. When you can approach your body size with the kind of indifference that you view something else, like the size of your feet, or your height, you begin to approach body neutrality. I'm not all the way there, but I get glimpses.
(For the uninitiated, ELI5 is often used for Explain it Like I'm 5. So this is the ELI10 version. I think this explanation might also work for a five-year-old, and if I get the opportunity to test it, I will follow up.)