Go to your Listing Preset, and expand the Template section. Look for your image placeholder in the preview screen:
Right click on the image placeholder, and look for Image Properties:
By default the image is set to relative scaling, meaning if you adjust the Width, the Height will follow automatically. On the left under the Image Info tab, you will see fields for Width and Height.
Try adjusting the Width to 250.
Success! Save your preset and your images will appear this size on any new listings.