On Windows, labels sizes are provided in mm because while many companies call their labels "4x6" or another size, they are in fact different sizes (100x150, 100x155, 100x160). 


So the easiest way to set the size is using millimeters vs 4 1/16" for example.



You can easily convert a size from inches to millimeters on google. 


Please refer to this link:

https://www.google.com/search?q=1+inch+to+millimeters