Unfortunately we have dropped support for non-proportional resizing in the current version of Resize. The "constrain proportions" checkbox is no longer in the Resize module. This was an option that was available in the early versions of Resize (Genuine Fractals). This means you will not longer be able to distort or stretch/squeeze your image to fit a particular document size within the Resize module. If your original document size is not proportional to your final size you will need to crop the image in the Resize app prior to resizing the image.
However, if you do need to resize your image to a size that is not of equal proportion, you will be able to change the proportions of the image in the Layers module prior to sending the image to Resize. Here is a video from our Senior Product Developer Dan Harlacher on how to do so: