Hello,
The answer is "planet, solar system, galaxy, then universe".
Reason:
A planet would be the smallest because it orbits the sun, then it would be solar system because a solar system is everything that orbits the sun, then it would be galaxy because it contains the solar system, then it would be universe because it contains everything in space: planets, stars, galaxies, comment's, asteroids, etc...
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit