Logged in users can set their default viewing size of thumbnails in the “my preferences” area under “files”. The default size for users that are not logged in is 220 pixels. Logged in users can choose from widths of 120px, 150px, 180px, 200px, 220px, 250px or 300px.

Also under “files” in the “my preferences” area, one can set the size limit of images shown on image description pages. By default, if either the width or the height exceeds 800px or 600px respectively, then the image is reduced in size until it fits within those dimensions. This can be changed to the default size of 320x240px, 640x480px, 800x600px, 1024x768px, 1280x1024px or 10000x10000px, if preferred. The last choice will effectively display all images at 100% resolution.
text source