You would use "Fit Contents Proportionally" for the different ratios, say landscape or portrait.
The size of the photos being displayed is based on the size of the widgets photo area.
You can scale it up to any size, but that specific size is the basis for all your images.
You cannot have different sized photo areas inside of a single widget.
So if your image box area is 600 pixels by 400 pixels, all the images will have to fit inside that area.
Hope this makes sense.