Does this add new barrel items?

No. It changes the fluid amount used by generated fill and empty barrel recipes.

Is this safe for UPS?

Yes. The mod is data-only and has no runtime script, tick handler, hidden entities, or fluid simulation.

Is this safe for existing saves?

Yes. You can add it to existing saves. Existing filled barrel items will empty into the configured amount after the mod is enabled.

Does this affect achievements?

Like other mods, enabling it marks the save as modded for achievement purposes.

Does this require Space Age?

No. It only requires Factorio 2.1.

Does it work with modded fluids?

It should work with normal generated barrel recipes for modded fluids, and it also checks recipes in the fill-barrel and empty-barrel subgroups.

Can I change the capacity?

Yes. Change the startup setting "Fluid per barrel". Vanilla is 50; the default here is 10,000.