Why are galaxies different sizes?
Depends on the black hole in the middle of the galaxy.
Yes, in each galaxy lies a black hole. Some galaxies have smaller ones, making the galaxy smaller in size, but some have huge black holes, making the galaxy bigger in size. This is one of the reasons why scientists are finding it hard to locate a planet in the "habitable" zone, because the black holes could create a problem if, lets say two galaxies were too close together, then that galaxy which is being pulled apart by the other, will not be a safe place to be.
Of course there are other reasons as well, like how large the sun is and where the planet is in accordance with that sun. Anyway, I hope that helps you and there might be another reason as well, but I am not an expert.