How to use crop node ?

.   Could somebody explain how  to use crop node please.  

For example I want to  cut/crop  a single 1024x2048 image into  two.

One  in between  200th pixel  and 456  in x dimention.    And another in between 456  and 584 pixels .  Basically cropping off two side by side textures from a single intital image : 256x2048 and 128x2048 .

What should I do?    Documentation says I should set input image size in pixels , right?  

When I do it  I get cropped part  somehow away from what I see doubleclicking  a node before the  crop one. As documentation suggests .   I am totally lost