FYI - If you have a monitor with a native resolution greater than 4096 in width, like an iMac 5k, or the ultra ultra wide 5120x1440 display, you may have found that you get an S:101 error if you set a custom resolution to match your display.
This is not a Shadow limitation, but a limitation of the Nvidia encoder that is used by default. If you enable the low-bandwidth mode setting in your client, you can run Shadow at much higher resolutions. This setting leverages the h.265 encoding which requires a little more horsepower on your client, but having native resolution is worth it!
Just make sure to set low-bandwidth mode on before setting the custom resolution or you will get a S:101 error.
Enjoy your super high resolution Shadow!