Calculating the straight distance of non-straight polyline in QGIS

Question

I have a GIS layer of bus stops (MultiLineString), which contain the paint lines that are painted on the asphalt.

The complete bus stop element is in a single MultiLine element. If I select it:

I need to know the length of the bus stop to know the total number of meters to act on the adjacent curb. Is there a method to achieve this?

I have 100+ bus stops. Some examples:

Answer 1

The query I was able to make with the hints in the comments above and that worked for me:

WITH 
  bbox AS (
    SELECT id, ST_OrientedEnvelope(the_geom) as geom
    FROM mytable
  ),
  corners AS (
    SELECT id,(ST_DumpPoints(geom)).geom AS geom
    FROM bbox
  ),
  distances AS (
    SELECT a.id, ST_Distance(a.geom, b.geom) AS distance
    FROM corners a, corners b
    WHERE NOT ST_Equals(a.geom, b.geom) AND a.id=b.id
  )

SELECT id, MAX(distance) as distance
FROM distances
GROUP BY id;