You want to measure the OD of a male threaded fitting. These are DIN metric O-ring fittings. Here is a chart of the ODs for DIN Light:
learnfittings.com
The OD is going to be 12, 14, 16, or 18mm. The thread pitch will likely be 1.5mm per thread. I doubt you have DIN Heavy fittings.
I post a chart from DiscountHydraulicHose.com on the back of my workshop door. They have all the fittings you need, plus don't forget to buy some extras, including plugs when you order. Or just go to a local shop if you have one.