In Australia, we measure them across the base first, then the height.
So, standard cupcakes are 50mm across the base, but you can get ones a smidge bigger at 55mm.
And you can also buy slightly deeper ones too.
So, I would advise measuring across the base, and then the height, and look online for a cupcake liner supplier who also lists these figures. They might also have an identifying stock number. Here we have number like 500, 700, 440 and so on, and...