Olimex power jack
Our boards usually use a DC barrel jack with a 2.0mm thick inner pin. The outer hole diameter is 6.3mm.
The standard power supply jack used in most of the OLIMEX boards has positive voltage in the inner pin:
The corresponding power supply plug (or female jack) fits the above DC barrel has 2.1mm inner diameter and 5.5mm outer diameter. The length of the male plug should be between 10mm and 14mm.