Tell her a partial truth

"Megan, I need to be honest with you, my shrinking is connected to your growth, but it's not your fault. In fact, it's not even a bad thing." You start.

Megan looks back at you, her concern shifting to confusion. "What are you talking about?"

You pause, reassuring yourself that if she thinks it's weird, you can always undo it with the stone.

"Alright, this is something I've never told anyone before, because frankly it's weird and embarrassing, but you're my best friend so I figure if there's anyone I can trust it's you, so here it goes." You take a deep breath.

"This, what's happening right now... is kind of a fantasy of mine. I think, ever since that short period of time where you outgrew me freshman year, I kinda realized I really liked taller girls, or at least girls that are taller than me. In fact, I kinda developed a crush on you. But when I eventually passed you in height I kind of stopped thinking of you as anything more than my best friend. But yesterday, when you showed up here taller than me again, I realized that the feelings I had for you back then never went away, and that I'd only been missing them."

"So I guess what I'm trying to say is, I like you, Meg. And you being taller than me, it's definitely not a bad thing, in fact it's quite the opposite. Its like a dream come true. And while I know this is probably completely out-of-nowhere for you, I wanna be more than just friends, if you're willing to try."

You finish your mostly-true, only-a-small-fib-by-omission monologue and catch your breath. Your eyes scan Meg's face, who's wide-eyed expression looks like she's just watched a pig fly over a frozen hellscape.