Technically speaking, short people find out last that it's raining, amirite?

Not if they are standing next to a tall person who is sitting.

Depends also on the height a short person is standing on. A short person on a ladder or a second floor balcony will find out sooner.

Depends on if it's hitting them on the head, vs just staring out the window.

Depending whether you're standing right next to the taller person or nah

Not necessarily. Sometimes you see surrounding raindrops falling on the ground before you feel something wet on your body, and the shorter person is closer to the ground so the light reaches his eyes first.