Hello and welcome to Microsoft Community!
I understand your problem, you are printing with custom sized specifications, the IPP class driver does not show the paper size you set in the print menu and you are not able to get the complete most out of your printing capabilities.
The IPP Class Drivers allow Microsoft to provide baseline functionality for printers without installing third-party drivers or utilities.
But you don't have to use this driver, you can download the corresponding driver for your printer model (Epson WF-7820).
For Epson printer drivers, you can visit the following link: EPSON Download Center
Enter the corresponding model number to download the driver.
Alternatively, you can give feedback on your printer problem to Epson technical support, who may be able to give a more professional explanation.
Epson WorkForce Pro WF-7820 | Support | Epson US
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
I hope my advice will help you.
Best Regards!
Leo - Microsoft Community Support Specialist