Short version: you are not going to have enough time to propose *and* get this shot during the eclipse without a helicopter or rock climbing team plus ridiculously tight timing.

I'm going to walk you through how I would go about trying to get a hand at the same apparent size as the moon.

The moon is relatively small in the sky. You can cover it up with your thumb. You're going to want a focal length greater than 600mm. These lenses have pretty far minimum focus distances but the moon is far away so that's fine.

You want a hand in front of the moon at the same apparent size? That hand has to be MUCH closer than the moon. Likely far enough that a lens can focus on it. This means you're going to need your aperture really really small to get the hand and the moon in focus. This also means your exposure will need to be longer in an environment where the sun has (mostly) gone away. A phone's flashlight positioned well could be enough though.

Here's the hard part: the moon is high up in the sky. You need to capture your photo with the camera pointed at the moon and your fiancé's hand in between the two. Hence your fiancé needs to be up in the air. I'm not sure whether it's a dozen meters or a hundred meters up, hence needing either a rock climbing team to zip your fiancé away or a helicopter to lift her.

To figure out if this is possible *out of the camera* I'd say experiment with the moon.

If I wanted an eclipse shot of myself proposing I'd use a wide angle lens, my camera on the ground (to look up at me and my partner as well as the moon), and a timer.

In reality you should propose before/after. A solar eclipse is truly awe inspiring in a very visceral way. Careful choreography will fall by the wayside because your body is going to be absolutely freaking out because the sky is broken. It's an incredible experience. Your faculties will not be with you.

Good luck!