Manual:$wgThumbLimits

Details
Users can select a default image thumbnail size in their preferences, which is used for all images displayed as thumbnails that lack an explicitly defined size. In order to reduce disk usage, users can only select a value from this list.

Default
and

Example
In order to reduce disk usage, the number of different values selectable from a list may by reduced. Put these settings in your LocalSettings.php:

If you change this array, you also want to change the   for thumb sizes on pages. Each thumb size set is automatically associated with an integer value that may be used for the "thumbsize" user option. This value always starts with "0" for the lowest thumb size set in the array and increases by 1 per additional thumb size defined, e.g. if five different thumb 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!