You can find a list of all configuration settings supported by maps in the "DefaultSettings.php" file ("Maps_Settings.php" until Maps 6.3.0). To see which configuration settings are supported by the version of Maps you have, look in its "DefaultSettings.php" file, which can be found in the root directory.
If you are not familiar with MediaWiki configuration, consult the "LocalSettings.php" manual. Do not edit "DefaultSettings.php" directly, as this will likely cause problems when you upgrade.
As of version 6.0 of Maps you will need to load the Maps_Settings.php file (not DefaultSettings.php) before changing the default settings. Example section in your "LocalSettings.php" file:
wfLoadExtension( 'Maps' ); require_once __DIR__ . '/extensions/Maps/Maps_Settings.php'; $egMapsDefaultService = 'leaflet'; $egMapsMapWidth = 500;