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 the amount of disk space taken up by thumbnails of many different sizes, limits can only be selected from this list.

This setting also determines the list of thumbnail links which are provided on the image page, just below the image.

Default
and

Example
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:

If you change this array, you also want to change the default user option for image sizes on image pages. Each image size set is automatically associated with an integer value that may be used for the "imagesize" user option. This value always starts with "0" for the lowest image size set in the array and increases by 1 per additional image size defined, e.g. if five different image sizes were set the lowest value for the user option is "0" while "4" is the highest. If an invalid value is set the wiki will break!