It has been pretty normal for me to see the Knockout specified similar to a hole callout.
Just give the measurable diameter followed by Knockout and a direction in reference to the view, if it matters (1" Knockout Up). Calling out the conduit size opens the door for confusion because a 1/2" conduit fitting does not use a 1/2" diameter knockout. So are you passing a fitting through the knockout or just the conduit, hmmm.
Also if your knockout requires more specs than that, for UL or Nema Certification or something like that you should specify the complete criteria for the Knockout as it is stated in the applicable standard, which has criteria for integrity of the knockout and possibly enviornmental issues (water tight, raint tight, dust proof...).