Hello Nicola,

You can define the size of an image in the particular function you will use for displaying images. You can use <%# GetImage("Image", int width, int height) %> for instance (if the image is stored in Direct uploader form control) and set the size to fit the thumbnail or some other functions depending on a form control which is used for those images.

Complete list of all available functions you can use in transformations can be found in Context Help - Methods in transformations.

Best regards,
Filip Ligac