It obviously affects all space, inside galaxy or outside of it. Let's just check the rough math, not even bothering about the fact that expansion was not linear. The universe expanded to around 100BLY in about 10BY, i.e. got ten times bigger than its potential light cone. So, for every "initial" 1km, there is additional 9km in 10BY timespan, or 0.0009mm per year, or 0.00003nm per second. The general ratio is around 1e-17 per second per length unit.

Proton size is around 1e-15m, Planck size is around 1e-35m. A thousand Planck units per second is not that much, really. And on sub-milliseconds it just loses its effect completely, due to that Heisenberg guy. (If we take my bullshit math seriously, I mean.)

Of course, if you calculate out far enough time, you would eventually end up with the observable universe getting smaller (because the stretching of spacetime would take away more space than was added by light reaching us from further away), down to our galaxy, and eventually everything getting torn to shreds, even down to protons...

In the "near" term, the effect is so small that we have much more to worry about from the sun expanding in a few billion years than the Big Rip.

I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart, and over time 1 light year expands to be 10% longer than it was before... and also, everything in the universe expands to be 10% larger in all spatial dimensions... then the distance in meters between the two points hasn't changed, because meters grew too.

Really? These structure constants never involve any units of distance? Isn't distance changing at all levels?

If I have a meter stick, and then space expands by 10%, you're saying that my meter stick will end up being 0.91 new-meters long, yes? Wouldn't that require the structure constants to change, to allow for the same molecules to take up less space?

Let’s come at this from the other direction: if everything including the space between atoms and galaxies expands by 10%, how do you tell the difference between an old-meter and new-meter? For this to be observable, something has to still scale by the old-meter, or you wouldn’t be able to observe a difference at all. That something would be the physical constants with units, like the speed of light. This means the electrical force governing the chemical bonds in your meter stick will still try to restore the distance between them to their original old-meter distance, but in new-meter units.

Then if you define the actual meter by the length of your meter stick, or the physical constants, you end up getting the same meter that everything not macro-scale “snaps back” to.

The meter-long stick you put into that space to measure it is still one meter long at some time in the future because the force that pulls the space apart (dark energy) doesn't pull on the matter that makes up the meter stick strong enough to overcome the forces that hold the stick together. But even if it did, the idea of one meter hasn't changed its definition. It's just the tool you used to measure one meter has broken.