falling disk, size function
 

Hi,
im simulating a falling disk in a room (3D with dynamic mesh), i've meshed my geometry by size function in Gambit, i have read the mesh in fluent and i want to use dynamic mesh.
in dynamic mesh menu in remeshing part there is a menu called size function , is there any one who can tell me how can i use that size function?
i want my mesh remain small near the disk so i want to use size function in fluent but i dont know how!
{what does
resolution
variation
Rate mean in that menu}
and how can i force the size function to remain with the disk as it falls?
tnx!