Communicate to vendor the UOM size, e.g. Box/50, Box/500...?

I am in SL 2015 > Purchasing. On the Help page, examples of UOM are BOX, CASE, CARTON, etc.

In the event that the vendor sells different size boxes, let's say BOX/50 and BOX/500, it wouldn't be sufficient to enter a UOM of 'BOX'. So, where do I communicate to the vendor on the PO the box size? Should I just create two UOMs called BOX/50 and BOX/500? 

Thanks!