Under settings, display, Output size. My 2nd display is a 1280x1024 turned to portrait with audio mixer below it. I have the output at the top as 1024x575 - kind of half size of 1920x1080.

I wanted to verify that this section is just screen output?

I am recording at 1080i/59.94. What is the correct setting for the display output?
Does it make any difference on rendering or stress on the GPU?

Thanks for confirming.

I could not tell by the instructions if the selected output size had anything to do with the load on the gpu. Hence, having it set to 1920x1080 and then outputting to the "full Screen" additional display at a lower resolution than required vs just setting it to what is used such as in this case 1024x576. By testing, I could visually see difference in the 2nd display when output was set to 160x120. I didn't see much of a change in rendering degradation.

I use a 1920 x 1200 main display for the inputs and previews. The additional display is for the sound tech to can see what's being output and also visually see the audio mixer below it.