The size of the result appears in the 'New Width' and 'New Height' fields. To change it, you can
Alongside each of the labels 'Zoom', 'New Width' and 'New Height' you can find a radio button. By these you can mark which of the three settings should be kept, when the source changes by cropping or opening a new image, which is especially important for Batch Processing.
For example, if you need results which are all 1000 pixels wide, mark the radio button at 'New Width' and type '1000' into the box. If you crop or replace the source now, the zoom is changed accordingly to produce a result of the desired width. When you change the zoom directly, the 'width' radio button is cleared and 'zoom' marked instead.
You can even force a fixed width and height by marking both the width and height radio button. In this case, the result will be distorted to get the desired size.