Set Thermal Printer Preferences

Open Printers & Scanners in Windows.

Click on your thermal printer in the list of printers and scanners. Then, click the Manage button.

Click the Printing preferences link.

Select the Page Setup tab. Then, select the correct label size from the Stock drop-down menu.

Again, the tab and menu names may differ for your specific printer. Reference your printer documentation for specifics.

Custom Label Size

Most thermal printer's preferences allow you to define a custom label size. This is useful when you are printing on a label size not listed. For example, NetStamps rolls have labels that are 1.31 " x 1.31".

Click Apply, then OK to save your preferences.

Open the Terminal applications on your Mac.

Terminal is located in Applications > Utilities, or you can use your Mac's Spotlight Search. 

Copy and paste the following command into the Terminal window after the $ prompt:

cupsctl WebInterface=yes

Press Return on your keyboard. 

The cursor will move to a the next line in Terminal. So, as long as you don't get any errors you are good to go! Quit Terminal and return to your browser.

Reload the browser with the http://localhost:631/printers address in the URL.

CUPS will open and display the list of installed printers.

Click the printer name of your thermal printer to open the printer settings.

Select Set Default Options from the Administration drop-down menu.

Select the correct label size from the Media Size drop-down menu on the General tab. Then, click Set Default Options.