in the file /conf/_advanced.php , look for:

/**
 * Thumbnail size definitions.
 *
 * NOTE: this gets used for general resizing, too. E.g. in the coll_avatar_Widget.
 *
 * type, width, height, quality, percent of blur effect
 */
$thumbnail_sizes = array(
			'fit-720x500' => array( 'fit', 720, 500, 90 ),
			'fit-640x480' => array( 'fit', 640, 480, 90 ),
			'fit-520x390' => array( 'fit', 520, 390, 90 ),
			'fit-400x320' => array( 'fit', 400, 320, 85 ),
			'fit-320x320' => array( 'fit', 320, 320, 85 ),
			'fit-160x160' => array( 'fit', 160, 160, 80 ),
			'fit-160x160-blur-13' => array( 'fit', 160, 160, 80, 13 ),
			'fit-160x160-blur-18' => array( 'fit', 160, 160, 80, 18 ),
			'fit-160x120' => array( 'fit', 160, 120, 80 ),
			'fit-80x80' => array( 'fit', 80, 80, 80 ),
			'crop-80x80' => array( 'crop', 80, 80, 85 ),
			'crop-64x64' => array( 'crop', 64, 64, 85 ),
			'crop-48x48' => array( 'crop', 48, 48, 85 ),
			'crop-32x32' => array( 'crop', 32, 32, 85 ),
			'crop-15x15' => array( 'crop', 15, 15, 85 ),
			'crop-top-80x80' => array( 'crop-top', 80, 80, 85 ),
			'crop-top-64x64' => array( 'crop-top', 64, 64, 85 ),
			'crop-top-48x48' => array( 'crop-top', 48, 48, 85 ),
			'crop-top-32x32' => array( 'crop-top', 32, 32, 85 ),
			'crop-top-15x15' => array( 'crop-top', 15, 15, 85 ),
	);