The form width you are trying to create is wider than the T6215 is able to print on. The maximum page width that you can print is 13.6 inches, if you try to create a custom size paper wider than 13.6" Windows will not show you this form within the T6215 printer driver. As you can not physically print 21" wide you do not need to define a paper size this wide. Simply create a paper size which reflects the maximum print width or 13.6" You will then be able to print the maximum width the T6215 is capable of and also see this form in the printer driver.
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The form width you are trying to create is wider than the T6215 is able to print on. The maximum page width that you can print is 13.6 inches, if you try to create a custom size paper wider than 13.6" Windows will not show you this form within the T6215 printer driver. As you can not physically print 21" wide you do not need to define a paper size this wide. Simply create a paper size which reflects the maximum print width or 13.6" You will then be able to print the maximum width the T6215 is capable of and also see this form in the printer driver.