Per AWS D1.1
The minimum diameter of the hole for the plug shall be no less than the thickness of the part containing it plus 5/16(8mm) preferably rounded to the next greater odd 1/16". The maximum width shall equal the minimum width plus 1/8"(3mm) or 2-1/4 times the thickness of the member,which ever is greater.
For slotted holes it is the same as above except the length of the slot shall not exceed 10 times the thickness of the part containing it.
The minimum center to center spacing for plug welds shall be 4 times the diameter of the hole.
The minimum center to center of slot welds (traverse direction (this way II) shall be 4 times the width of the slot. Longitudinal direction (--) shall be 2 times the length of the slot.
Hope I do not confuse you with the (II) and the (--).