this would be like in blender where you have the dimensions of a selected image in the the tool options,and you can just adjust the slider to the left right top or bottom to reduce the size. 

For when you have an image with many poses and you need to focus on only one.



Pureref also does this

Select an image ,press C and hold => drag a rectangle across the image,it hides everything outside the rectangle when you let go.

When you RMB or use a shortcut you can undo the crop.