It depends on what the child and the parent are, not all nodes have a size of their own.
For example, you can change the size of a sprite by modifying its
Then, you need to know the size of your
Area2D: this depends on the collision shape you used. If it's a rectangle, you would need to adjust the scale of your sprite in such a way the resulting size is the same as the rectangle. Finally, your sprite needs to be centered on the
Probably something like this:
# Assuming the area has a child CollisionShape2D with a RectangleShape resource
var area_size = area.get_node("CollisionShape2D").shape.extents * 2.0
# The size of a sprite is determined from its texture
var texture_size = sprite.texture.get_size()
# Calculate which scale the sprite should have to match the size of the area
var sx = area_size.x / texture_size.x
var sy = area_size.y / texture_size.y
sprite.scale = Vector2(sx, sy)
But this may be wildly different depending on which nodes you have and how they are structured.