Hi Paul,
I have two sets of T21 plans. The Marine Modelling ones, which do not show the portholes at all. I also have the Jecobin HMS Active in 72nd. Whilst the side profile shown is the Port side, the portholes on the Stbd side are shown as hidden detail.
They measure approx 5-6mm diameter, depending on which one. They would of been all the same size I believe....I would be tempted to go with 5mm, which gives a true size of 359mm.
I seem to remember that one of them was in the CO's bathroom. And yes, he had a real bath, the only one aboard.
Nige