Please can someone answer the following: What does a fixed volume mean? Do solids have the smallest volume?

You are most likely doing thermodynamics. The simplest example would be the Kinetic Gas Theory also known as the Ideal Gas Equation. ##PV=nRT##

In a fixed volume like a metal container, gases can exert pressure at different temperatures (Gay-Lussac's Law).

In a changing volume, you will have a balloon or a piston where the volume changes via gaseous expansion.

Solids do have a smallest volume. Try cooling the object to absolute zero . In theory, particles would have zero kinetictic energy and would not vibrate or move at all. Scientists are not exactly sure it this is the only thing that would happen.

I have heard that the International Space Station will be adding a machine that would cool things to record low temperatures (very close to ##0K##). They need low gravitational influence to obtain lower temperatures.