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