Element Size allows you to specify the element size used for the entire model. This size will be used for all edge, face, and body meshing.
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.
Note: When you set Physics Preference as Explicit, the default Max Size is equal to Element Size. That is, to get a uniform mesh, the volume max size is automatically set equal to the surface max size.