How do I change the Printer Default Settings?

I am using an iMac (macOS Sierra, version 10.12.3), trying to print to a Toshiba 4540c.

My preset "default setting" are currently set to have a custom sized paper (8.76 x 11.34).

When I print with these settings, the document ends up printing sideways and half of my page gets cut off....basically, I have to go back to the computer and reprint, which with the printer being a good distance away from my office can be quite frustrating.

Most times, I remember to click on the Presets Dropdown menu and select the correct preset, but I would desperately like to change the actual default setting to 8.5x11. I have no idea how this came to be the default size, as I recently started in this position, and all of the setting are from the previous user of the computer.

I saw many people suggesting to go to the cups.org website and adjusting the settings that way, which I did, but nothing changed!

I am so confused. I feel like there has to be an easier way to change the default settings. Does anyone know how to go about changing the preset default print settings?