any size you like! ;D
give about 20-25mm space between the case and the rad, to allow for airflow thru the APE.
make sure the spacers have the M4 threaded stud, as this is what bolts to the APE, the threaded hole is normally the same thread on that type of stand-off, but it can be anything, cos the bolt heads will be on the inside surface of the case side panel.
have fun