Thanks for the reply. Can you provide an indication where to find this functionality? I can only find the option to crop to a fixed ratio. What I want to do is take screenshots for a user guide, and crop each image it to be the same size (400 x 250 pixels).
First click the crop preset dropdown in the crop tool option bar and select "No Restriction". You might have 'Photo Ratio' or some other preset ratio selected there. After choosing 'No Restriction' enter the values you want in the width and height textfields. Commit and image size will change to that.
Do let me know if there is any confusions.