Noun
すいしょう
この水晶は大中小の三つの大きさがあります。
This crystal comes in three sizes; small, medium, and large.
A water crystal is just a crystal. I guess crystals are clear and look kind of like water, yeah? That's why it's being referred to as a water crystal, making it really just a crystal.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.