Ah... I never use the Auto settings screen.

To begin with, I would suggest you switch to the Manual Settings screen and import the above Auto settings. Then reduce Irradiance caching to 20 or less and increase the Min shading rate to 2 or more, and bucket size to 32, or 64 if your PC will take it. This should make your test renders quicker, but you should still be able to see what's happening with your changes to lighting and shaders.

Once you've set the aspect ratio and full size of your render, there's no need to change dimensions for the test renders... you can just select Half size, or use the "Fit in preview window" option, if your Preview window is a reasonable size. 

The dpi setting is irrelevant, by the way. It makes no difference at all to the render.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)