Ansys Meshing / Use Advanced size functions
‘Use Advanced Size Function‘ option gives easy control over sizing functions.
‘Use Advanced size functions’ control the following properties:
• Angles between normal for adjacent mesh elements (curvature)
• Number of mesh elements employed in the gaps between two geometric entities (proximity)
• Gradation between the minimum and maximum sizes based on a specified growth rate (all size functions)
Another option of sizing group are explained in this blog
Volume Meshing:
• General Sweep (Surface mesh only)
• Thin Sweep (Surface mesh only)
• Hex Dominant (Surface mesh only)
• Patch Conforming Tetrahedron
• Patch Independent Tetrahedron
• CutCell
Surface Meshing:
• Quad Dominant
• All Triangles
Specifying Size Function Options :
The value of the Use Advanced Size Function control (Off, On: Proximity and Curvature, On: Proximity,On: Curvature, or On: Fixed) determines which size function options are available in the Details View. The default is Off, unless Physics Preference is set to CFD, in which case the default is On: Curvature.
The following options are available in Ansys meshing:

 Curvature Normal Angle
 Proximity Accuracy
 Num Cells Across Gap
 Proximity Size Function Sources
 Min Size
 Max Face Size
 Max Size
 Growth Rate
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Curvature Normal Angle

 Curvature Normal Angle is the maximum allowable angle that one element edge is allowed to span another element edge. It is active only when Use Advanced Size Function is set to either On: Proximity and Curvature or On: Curvature.
 You can specify a value from 0 to 180 degrees or accept the default. The default is calculated based on the values of the Relevance and Span Angle Center options.
 Generally for Coarse: 91 to 60 degrees, for Medium: 75 to 24 degrees, & for Fine: 36 to 12 degrees are recommended.
 When Capture Curvature is set to ‘yes, and angle is smaller than setting up angle then it is automatically generating more fine elements in the curved area. fig shows that.
Default Ansys mesh 
Capture Curvature is set to ‘yes’ 
Proximity Accuracy
 The proximity size function allows you to specify the minimum number of element layers created in regions that produce “gaps”
in the model.  Proximity Accuracy allows you to control the accuracy level of proximity size function calculations that are performed
between pairs of entities.  The Proximity Accuracy control is available only when Use Advanced Size Function is set to either On: Proximity and Curvature or On: Proximity. You can specify a value from 0 (faster due to calculation but less accurate) to 1 (slower due to calculation but more accurate). The default is 0.5.
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Num Cells Across Gap
 Num Cells Across Gap is the minimum number of layers of elements to be generated in the gaps. Available only when Use Advanced Size Function is set to either On: Proximity and Curvature or On: Proximity.
 You can specify a value from 1 to 100 or accept the default. The default is calculated based on the value of the Relevance option. If Relevance is 0, the default of Num Cells Across Gap is 3.
 The value of Num Cells Across Gap is an estimate; it may not be exactly satisfied in every gap. CutCell The value of Num Cells Across Gap is approximate for meshing. For example, if you specify 3 cells per gap on a narrow face, the final mesh may contain anything between 24 cells across the gap.
Min Size
 Min size is a global minimum allowable element size. It is active when Use Advanced Size Function is on.
 For Min size Specify a value greater than 0 or accept the default.
Max Face Size
 The Max Face Size specification represents the global maximum allowable size of the elements created by the free surface meshes of the
supported methods.
Max Size
 Max size is the global maximum allowable element size. It is active only when Use Advanced Size Function is on.
 For Max size, Specify a value greater than 0 or accept the default.
Growth Rate
 Growth Rate represents the increase in element edge length with each next layer of elements.
 For example, a growth rate of 1.2 results in a 20% increase in element edge length with each next layer of elements.
 It is available when Use Advanced Size Function is on. For Growth rate, Specify a value from 1 to 5 or accept the default. The default is calculated based on the values of the Relevance options.
Another option of sizing group are explained is this blog