05AB1E, 13 bytes

ŒʒlêAå}éΣág}н

Outputs an empty string as falsey (could have been 12 bytes if we would be allowed to output -1 as falsey with ŒéΣág}.ΔlêAå).

Try it online or verify all test cases.

Explanation:

Π      # Get all substrings of the (implicit) input-string
 ʒ      # Filter this list by:
  l     #  Convert the current substring to lowercase
   ê    #  Uniquify and sort all characters in the string
    A   #  Push "abcdefghijklmnopqrstuvwxyz"
     å  #  Check if the lowercase alphabet is in the uniquify-sorted substring
 }é     # After the filter: sort any remaining substrings from shortest to longest
   Σ    # Then sort it again by:
    á   #  Leave just all the letters of the substring
     g  #  Pop and push the length
   }н   # After the sort-by: pop and leave just the first
        # (or an empty string if the list is empty)
        # (which is output implicitly as result)