Element Size allows you to specify the maximum surface element size used for the entire model. This size will be used for all edge, face, and body meshing. You can parametrize Element Size. When you set Adaptive Sizing to Yes, mesher do not follow the Element Size.
For solids, the Element Size is calculated as Bounding Box Diagonal * Bounding Box Factor.
For sheets or solids/sheets, the Element Size is calculated as sqrt(Average Surface Area) * Surface Area Factor.
Element Size is calculated based on the Physics Preference you select. The Element Size calculation for each physics preference is as follows:
When Physics Preference is Mechanical, Nonlinear Mechanical, CFD or Hydrodynamics,
Element Size = Assembly Diagonal * Bounding Box Factor
When Physics Preference is Explicit or Electromagnetics,
Element Size = 0.5* Assembly Diagonal * Bounding Box Factor
Note: When you set Physics Preference as Explicit, the default Max Size is equal to Element Size. That is, the volume max size is automatically set equal to the surface max size to create a uniform mesh.