By default, the plugin will always use the backstretch sized image if the featured image is large enough. If it isn’t, then it will use the large size image instead. If you want to conditionally prefer the large image, even if a backstretch version is available, you can use the display_featured_image_genesis_use_large_image filter–this and similar filters are documented on the FAQ under the “How can I change how the plugin works?” question.
The setting to remove the number of pixels for the height is because of how the backstretch image script works: by default, it basically checks the browser window height, and sets the backstretch image to be that tall, in order to fill the screen. So the number will totally vary from one device to the next. And this measurement doesn’t factor in elements that are already on the screen, such as a header or navigation menu, so the bottom of the backstretch image may end up being significantly “below the fold”, so to speak. So you could set this setting to account for your header/navigation, to make sure that the backstretch image ends at the bottom of the user’s screen. It totally depends on your design/setup as to whether this setting needs to be adjusted at all, or if you want to use it in combination with the Maximum Height setting, which affects CSS, but not the script. Hope that helps!
