"the element size" in ICEMCFD refers to?
 

There are settings for "global max element size" and "max element size" for a specific surface. What are those element size means? Are they means the size of "volumn", "sureface" or "line"?

Thanks!!

Look at the help navigator. It's written that "Global Mesh Size" apply for surfaces, volumes and prism layers. So I guess that element refers to the max length of your mesh elements!
If you want more information, you should have a look at the help navigator ;)

Sizes...
 

It is a unit size in 3D space... Each mesher interprets it a little differently.

The Octree mesher tries to get below this size (being above this size is the check that triggers subdivision) and you will find that the average edge length is about this size.

The Patch Conforming meshers translate this size to the perimeter curves to seed them with nodes (length divided by size gives the number of elements).

But yes, checking the built in help is a good idea. There is a question mark at the top right of every data entry zone and the help is searchable.