I was around 3, ~13 years ago, when I watched this movie, so I don't remember much.
First thing I remember is two people (brothers maybe), one of them definitely male, are driving home; the one that is definitely male is smaller and seems to be too young to drive, but it shows them driving down a road to go home while a spaceship with a green light follows them.
They get home and later that night the small boy gets bigger. We see this by it showing his feet suddenly pop out from under his blanket and his head staying in the same place. In the morning he notes how his clothes don't fit for some reason.
Sometime later they end up on an alien spaceship.