The easiest method:
player set scale to unlerp(-200, 700, player.y)
Where -200 is the y-coordinate where the player size will become 0, and 700 is the y-coordinate where it's scale is 1. Of course, you'll need to find your own values.
You can also wrap it in clamp to prevent the player from becoming too small or too large.
clamp(unlerp(-200, 700, player.y), 0.5, 1.1)