The automatic mode checks all Enviroment Texture nodes and picks the largest image's resolution.
If there are no Enviroment Textures, it just uses the old default.
Also, the sampling map now isn't limited to square shapes. The automatic detection uses the exact image size,
the manual UI option now halves the value to get the height.
A default aspect ratio of 2:1 makes sense, since this is what most HDRIs use.