My fluid 3D geometry is one cubic volume is subtracted some cylinders. I wanna the girds around circular cylindrs are finer while coarse far from cylinders. So, I think size function is needed. But, I dont know how to set up the parameters.

easy
in size function, let fixed option
The sources will be your cylinder surfaces
The attachment is you substracted volume
The start size is the element's size of your refined surface
The grow rate should be set to 1.2 (but it can be less for a low transition, or more for a rapid one),
Max size is the global size of the elements.