How can I edit a custom label size?

Would love to be able to at least start from a copy of an existing label if not be able to edit them. Having to start from scratch inputting all the dimensions is tough. Especially because you can't see what you input to an existing label to reference what to change. I end up having to keep separate record of my custom label sizes for when I have to make adjustments for say a printer that prints out offset from what it should be.