There is two ways (via or hole) to add a mounting hole in a PCB and the difference between the VIA and the HOLE tools can be confusing.
Here is the via tool:
Here is the hole tool:
A via is composed of a hole and a copper area surrounding the hole. But the main différence between vias and holes is the electric connection made between layers by the via. The HOLE tool just place a hole in the design without copper or connection. Depending on what the mounting hole is made for, you may add a VIA or a HOLE. The following capture shows the difference between vias and holes. On the left side of the PCB, holes has been added, and on the right side of the board, two vias has been placed.
As it is illustrated on the following figure, vias are isolated from copper pour:
Regardless the selected tool, the hole diameter has to be defined. The PCB editor is generaly configured in mils (or inchs) and the fastening specification is generaly provided in millimeters.
The following table is a quick references of most used diameters.