One of the most important values related to sizing is minimum size (Curvature Min Size and Proximity Min Size controls). Setting a value for minimum size that is too small may lead to an unnecessarily fine mesh and longer meshing time or robustness issues due to the mesh tolerances being smaller than the geometry tolerances (mesh falling into holes/gaps in the model). On the other hand, setting a value that is too large may mean that important features are not captured by the mesh.
The following notes may help:
Use local sizing controls to override global controls to either increase or decrease the mesh size as necessary.
Use the Mechanical Min Size Factor and/or CFD Min Size Factor to change the relationship between Element Size and the default min size. See Sizing for details.