Due to some limitations on the default printer settings on the Mac, there a couple label sizes that need to be customized. They are:
Rollo Labels 2 1/4 x 1 1/4
Zebra Labels 2 1/4 x 1 1/4
Here's a video on how to customize the labels. The video demonstrates Rollo printer, but the same principle applies to the Zebra printer as well.
After making any changes, close and reopen the ScanLister app.