Your ruler is absolute distances, like the speed of light. You can inflate or deflate the balloon and the number of marks on its surface won't change. But the time required for the light to travel between them will.
Stupid question - How do you know for certain it isn’t just that light is slowing down?
Relativity probably doesn’t work this way at all, but if light did actually slow down, would it be perceived by us as light moving at the same speed across a greater distance?
I asked Ann Nelson (look her up) this when I audited her cosmology class in grad school, thinking I'd come up with a great new idea. She said yes, it's equivalent to the speed of light changing, and then moved on.
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.
Not exactly. Remember, we're not timing the light's transit. We're observing a redshift of it's frequency which could not be caused by c simply decreasing over time.
