The phase of matter with a definite size and volume but not a definite shape is solid.

Here's why:

* Solid: The particles in a solid are tightly packed together in a fixed arrangement. This means they have a definite shape and volume.

* Liquid: Liquids have a definite volume but take the shape of their container because their particles can move around more freely.

* Gas: Gases have no definite shape or volume because their particles are spread out and move freely.