Under Mac environment, both Safari and Chrome do not provide an option to allow you to set a default printer and paper size. You can work around it using Firefox. Follow the instructions below.
Open Firefox.
Go to File Menu > Page Setup..
Select format for your receipt printer
Set the paper size to 72x2000mm
Select Save as Default in the Settings drop down menu.
Click Ok
After above steps, both Safari and Chrome will now use the new setting as default. (Remember to quit and re-launch Safari or Chrome for the settings to take effect.)