When CAD system is not executing any command, we can see the pickbox in the middle of the cursor,

as shown in the following picture,

If we execute certain command without selecting an object, for example the Move command, the copy command, the CAD system would prompt us to select object and at the same time, the cursor would disappear, only a pickbox prompting us to select object, as shown in the following picture,

Pickbox system variable

The default size of pickbox is 3 pixels, we can enter PICKBOX to see the current size or change the size by entering a new value, as shown in the following picture,

Then press Enter key,

Adjust the Pickbox size

Enter OP, press Enter key to open the Options dialog box, go to the Selection set tab, as shown in the following picture,