There are 2 ways to detect the dimensions of a label paper at hand:
Find informations about the product
... on the package the label paper comes
... online on the homepage of the manufacturer
... online on the homepage of some dealer
Take a measurement using a ruler
There is no need to find or measure every single value, some dimensions can be calculated from others.
For either strategy, a printout of this label dimension template file can be quite useful to take a note of every detail as you gather or calculate them.