The voxel size is the main control for the resolution of a Bifröst simulation. Large voxel sizes provide low detail, which can be good for quick tests and previews while you adjust settings. Small voxel sizes provide high detail and better accuracy for the final output.
The main setting for voxel size is Master Voxel Size in the Resolution attribute group of the main properties. This determines the voxel size used for the fluid simulation itself. It is also the base size used to voxelize the meshes that are used in a simulation, including emitters, colliders, and other objects. Note that when you change Master Voxel Size, you may also need to adjust the time adaptivity settings — see Optimize a Bifröst simulation for accuracy versus memory and computation speed.
In addition, there are extra controls for the size at which the meshes used in a simulation are voxelized. Every mesh property, such as an emitter or collider property, has a Voxel Scale attribute in its Conversion group which affects how the associated meshes are initially voxelized. Liquid simulations also have additional global controls for the size at which the voxelized meshes are combined: the global Voxel Scale attributes for colliders and other meshes are on the main liquid container, and for guided simulations, the Guide Voxel Scale attribute is in the global guide properties. Emitters have no global scale attribute.
For example, if you do not need detail on any collider, you can set the global Collision Voxel Scale to a value greater than 1.0 to save both time and memory. On the other hand if you do not need detail on specific colliders, you can set Voxel Scale to a value greater than 1.0 in the corresponding collider properties — this will not save memory in the end because the colliders will be combined at a smaller voxel size, but it might save time voxelizing large meshes with little detail.
In particular, if you set Voxel Scale in a collider property to a value less than 1.0 for more detail on a specific mesh, some of that detail may be lost when the colliders are combined at a larger voxel size. If you require high detail on specific meshes, you should use a global Voxel Scale less than 1.0, and a Voxel Scale greater than 1.0 in the mesh properties of the other meshes.
For aero simulations, objects are initially voxelized at (mesh Voxel Scale)×(Master Voxel Size), and combined at Master Voxel Size. Aero simulations also have a Voxel Size Render Factor that can be decreased to increase the detail used for rendering without increasing the resolution of the FLIP simulation itself.