Hi prashant,
Actually at BW side Short/medium/long descriptions can have length of 20/40/60 accordingly.
But at source or info object level text descriptions may not be maintained(stored) as exactly 20/40/60 length respectively.
So take respective text object value, check at info object level how it stored for short description, as my guess it will be below 20 chars. if you need 20 length of description then you need to change at source side or Bw side.
Thanks