Houdini-and-Chill

your primary driver is _what_ scale are you rendering at? everything else is just a scaling factor going from that to the preferred working scale of the solver. Pyro will handle very small values, and very large ones fine, so you don’t tend to mess with the working scale. 1m is 1m for example.
FLIP is notorious for being fiddly as small scale, so anything under 1m in real world size you tend to work in larger scales. Example, simming liquid pouring in to a glass, real size might be 0.1m, but you would generally work at 10x that in FLIP. But on the other end, if the scene is 10m or 100m or 1000m you would leave FLIP at normal scale.
Bullet similar deal. Anything with pieces under 1-2cm can be a pain, so we routinely work 10x in scale. But if your smallest piece is going to be decently sized you might not change working scale at all.
Vellum is roughly based around real-world, so pretty much never change this working scale.
Heightfields, it really just comes down to working in the scale that the solver/defaults are built around.
At the end of it, you’re really only talking about working in the scale best for the solver/technique, and then scaling to render scale for output/lighting.