If I want to simulate filling a glass with water, what scale should this be?
Should it be set up so the scale is like I'm filling a bath?
Should it be set up in the real world scale of a glass of water?

Thanks

I'd try it at real world scale first - remember that Houdini's default units are meters.

Thanks

That's what I took from reading the documentation.

I guess it is how hard it is to make the forces and settings work at that scale.

Think about the level of detail you need in the sim, and the fidelity of your mesh. For small scale stuff
like your glass, I'd treat that 10cm glass like it was a 1m, at least. Never had good results with fluid
simulators at real world size for small scale stuff.

Working at the larger scale will give you room to move with your sim spacing, and your meshing won't freak
out as most of the default VDB values, etc are not really designed to mesh something that small.
You'll find adjusting forces are a small price to pay for not crying about sim res, and also the meshing.