The size of screws you use it entirely up to you but you should decide this before starting the enclosure and PCB design. If you use a hole of diameter D then you must use a screw of a diameter for which D is a clearance diameter. In other words the screw must slide freely through the hole. The screw diameter depends on how big the board is and how many screws you plan to use to fix it in place. Perhaps the most commonly used screw diameters are M2,  M3 or M4 because hexagonal pillars threaded with these screw sizes are readily available. Also it is good practice to include the mounting and other holes (as well as stuff like logos and safety labelling) in the schematic so that they are pulled in to the PCB at the time you do Convert to PCB. For more on this please see (2.2) in (2) in: [https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a](https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a)<br> <br>