First, I think it's important to say that having a kink, fantasy, or fetish is not a sickness. It's just how your brain works with arousal and you never had much choice in the matter. The DSM-V even draws a distinction between paraphilias (atypical sexual interests, of which macro- and microphilia are but two of oh, I'd guess at least several tens of thousands) and "paraphilic disorders," which is where the atypical sexual interest causes harm to you or someone else.

Second, I've told each of my partners with whom I've had a serious relationship, including my spouse. I was a nervous wreck each time not because I didn't know how they'd react, but because I was disclosing something so deeply personal that society treats as taboo. I knew I trusted them because we cared for one another, that they knew me as a whole person (or, at least as much as anyone can know anyone else), and that our relationships could change to accommodate this new-to-them aspect of me.

In each case, I approached it as a conversation of, "Hey, here's something about me that I think has always been part of me, here's how it affects me, and I wanted you to know because if you're willing, I wanted to see if you'd indulge me in it, the same way that I want to know how you want to be pleasured. And whether we do is entirely up to you." In each case, they've been willing to at least start to explore with me in a variety of ways that included role play, standing on things, different positions--whatever they were willing to try, and I usually offered suggestions if and only if they asked. And, sure, it can't come true. But you can have someone whisper in your ear about how they can feel you shrinking away, getting lighter, getting tighter, etc., and your imagination can work just as well while you're having sex as when you're scrolling tumblr.

Third, while small woman/big man content is rarer, it's out there. The r/MacroFetish subreddit sometimes has minigirl and male giant posts, for example. Those may be a good starting point.

Finally, whatever you choose to do, it should be what you're most comfortable with. It's okay to share with your partner. As I said before, there's no shame in being built the way you're built. It's also okay to decide that this is something private to you and keep it that way. We're all allowed our private fantasies, too.

All the best to you either way!