Crop
Use the Crop operation if you want to resize your image to a given size (in pixels) and to centrally crop inside of it. You can use the gravity filter to change the crop region.
Examples
Original image

1280x832
326 ko
Crop 400x600

400x600
78,9 ko
Crop 300x300 + Gravity + Contrast

300x300
35,6 ko