PBI Canvas background dimensions.

The Image fit option is used to indicate how to stretch the image on the canvas, it's dimensions will depend on the canvas size shown in Canvas settings.

Normal - the image is not stretched, it appears in it's original size which may be larger or smaller than your canvas. If larger it will be cropped.
Fit - the image is stretched horizontally and vertically to fit your canvas. This will include shrinking as well as enlarging.
Fill - the image is stretched horizontally or vertically to fit your canvas. The image will retain it's original proportions and may be larger or smaller than it's original size.