From release notes of 6.1.1
"Gamepad Mode Scaling - A gamepad version of the Custom Scale setting can now be found in the gamepad Video settings. This setting provides control over the size of UI elements within gamepad mode. The keyboard version of this setting still only impacts UI elements outside of gamepad mode, and the two settings can be customized independently of each other."
On a Windows 10 pc:
There is no slider control for Gamepad Video settings for scaling, whether in Keyboard or Gamepad modes. Not under Settings/video/interface - Use Custom Scaling, nor under Gameplay/ Gamepad. I could't find any Video/Gamepad section in any other settings category.