There are a couple bugs with the volumetric slicing and also the Cp slicing. They are technically different things, even though we use the same GUI elements to specify both (and it is labeled for Cp slicing).
Right now, you can't control the extent or the resolution of the volumetric slicing.
Importantly -- the resolution of that slice has nothing to do with the quality of the solution. A potential flow code does not need/have a volume grid as part of the solution. Instead, the volume slice is purely a post-processing step on a solution.
I think we may increase the default resolution some and/or add some hooks to give some user control.
Rob