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 each has its own specific measurements.
Be careful though, as SOME wider based ones will not fit into your standard cupcake pans for baking.
Hope this helps.