From XnView Wiki

A fundamental operation, this one extracts a rectangle of specified size from the specified location in the picture. In other words, you may cut a strip of specified width off any or all edges of your image in one step.

The parameters are all measured in pixels. Hence you should check the dimensions of your image in pixels before starting. The X and Y parameters set the upper left point of the rectangle that will be retained. The height and width parameters set those dimensions of the retained rectangle. Note that the new size of the image in pixels will be those height and width values.