Sorry. I edited my reply above to fit with the actual information. It seems I was off in saying that this printer doesn't have that option, because it does. The HP Envy 100 e-All-in-One d410 Series Printer uses version 14 of the printer software. Drivers that work with version 14 and below of the software typically do support custom paper sizes. If there is no option for this feature in the driver, it may need to be reinstalled. I apologize for my confusion and for confusing other users.
This feature does have certain size limitations. What custom size are you trying to print on?