Hey
One easy way to extract the flow volume for this geometry using SimScale is by running an enclosure operation, as seen in this video.
There’s no rule set in stone for element sizing. Generally you want the local surface cells to be small enough to capture the details. Setting a limit to the local cell size assures that you will have a minimum resolution.
Also, keep in mind that the local element size of the surface is related to the inflate boundary layer refinement:
In order for you to choose good settings for your inflate boundary layer refinement, you have to conduct a y+ evaluation