Bear in mind that time is not universal. For the speed of light to change over time, it would change at different rates for observers in different reference frames, and this difference would be measurable.

As to the previous question about relative size. If the universe doubled in size, the objects within it such as atoms don’t also double in size. Gravity and the atomic forces don’t change, so eg the size of stable electron orbits or planetary orbits don’t double in size.