To measure an O rings its very simeple. Measure the ID (Inside Diameter) and the Thickess.
For example if we say the ID is 25mm and the Thickness is 2mm then you select a OR25x2 if you need it in Nitrile or OR25x2VIT if you need it in the Viton.
If you are having problems measuring the ID you can Measure the OD (Outside Diameter) and the Thickness and it's possible to work out the size from here. If you have an O ring that measures 30mm on the outside and is 2mm thick. To work out the ID it would be OD - Width - Width (30-2-2) = 26mm. So you would select a OR26x2