How do I make something shrink while it's heading towards a point?

You can use lerp and unlerp functions. For example:

Set bullet scale to lerp(1, 0.5, distance(bullet.x, bullet.y, player.x, player.y)/600)

At 300px from the player bullet scale will be 0.75, at 600px the scale will be 0.5

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Or another example with unlerp and based on Y coordinate:

Set bullet scale to unlerp(-100, 500, bullet.y)

As the bullet is traveling from y=500 to y=-100, its scale change from 1 to 0.

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Or simply use Sine/Tween behavior to change bullet size over time.