Under Mac environment, Imonggo currently supports Safari, Chrome and Firefox; however, Store v3 does not work with Firefox.
Both Safari and Chrome do not provide an option to allow you to set a default printer and paper size. So to work around it, follow the instruction below.
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.
After the 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.)