Manual:$wgImageLimits

Details
This variable contains an array of 2-element arrays. Each of these defines a width/height pair describing a maximum image size (in pixels). The image sizes defined in this variable are used on the user preference page to allow the user to specify an image size-limit when viewing image pages. In order to reduce disk usage, limits can only be selected from this list.

If you change this array, you might also want to change the default option for image sizes on image pages. By default which, by default, corresponds to 800x600 in this array.

If you are using a skin which has a fixed width for page content, you will probably want to make the maximum size equal to that width. Put these settings in your LocalSettings.php: