Tell her she needs to close her eyes.

The answer seems so obvious. If she can't see you writing, there's no way she'll be able to know the exact nature of the trick.

"My... uh... magic is a little shy. I just need you to close your eyes."

Diane purses her lips. "Oh is that all? I thought you were gonna ask me to take off my shirt or something." Without hesitating further, she closes them tight. "So how big am I gonna be?"

"T-that's up to you." You say, reaching for the notebook. With the closet as small as it is, you barely have any room to maneuver. "But... you need to keep your eyes shut the entire time or..."

"Or what? Bad stuff?"

You say something ridiculous, just to buy you time. "Or... your tits might deflate to tiny A cups."

"What? That'd suck." She pinches her eyes even tighter shut. "You tell me when I can open em. As for a size... I dunno... make em huge. Like... big as my head."

You swallow back a lump in your throat. She's not really taking this seriously... but at the same time, she seems awful.... sincere. It's like she wants to believe you.

"I dunno..." you say. "Boobs like that aren't usually real."

Diane scoffs. "Like I care. They could be fake as hell and I'd be down with it. Tits like that are expensive, I ain't got that kind of money lying around. If you hooked me up with those, I'd fuck you here and now."

You grow hard just thinking about it. "Size of your head... huh?" You look down at the notebook and puzzle over the dim lighting. You need a size to write in the notebook, but thinking about fucking Diane doesn't really help your math skills.

Terry. He obsessed over enough pornstars to give you a basic idea.

"So... uh... like a K cup?" you say.

"K?" Diane laughs. "They make em that big? I guess... I mean, I don't know much about that shit. Your... 'magic' ... needs a size?"

"Uh... n-no..." you lie. "I just... don't want you to end up bigger than what you want."

"Shiiit. If you want, you can just fatten them to the ground."

You chuckle. "I don't think there's enough room in the closet for that..."

"Fine whatever. K works."

You try to position yourself so you can write, and the corner of the notebook bumps against her boob. "S-sorry."

"It's all good," she says, shrugging. "You want to touch em' before they start growing? Or maybe you need me to suck you off to get em going...?" She smiles wryly. "Look. I'm not dumb. This is just a stupid excuse to make out, but if it's fun and it leads to fun sex..."

You're not really surprised to hear any of this... but now that you've gone this far. You could easily play this off as exactly that...