compare the crystal size to how rock is formed?

Answer 1

I'm assuming we are talking igneous in which case:
1. Big crystals take a long time and stable environment to form, so they mean the rock formed in the earth (ex. granite).
2. Smaller crystals formed more quickly, closer to or at the surface of the earth (ex. obsidian).

You can also get mixes of crystal sizes, if the magma cooled partially at depth within the earth, allowing large crystals to form, and then moved up through the earth and was extruded.