That's "intended", you didn't request any length in the string so. This code should have exactly what you want
Code:
on *:INPUT:#: {
  if (%length !isnum) set %length 3
  if ($len($1) >= %length) { 
    if ($fline(#,$1 $+ *,0,1)) { 
      var %nicks $v1
      while (%nicks) {
        var %nick $fline(#,$1 $+ *,%nicks,1).text
        msg # $effect(%nick) $2- 
        if (!%multi) break
        dec %nicks
      }
      halt
    }
  }
}

menu channel { 
  Nick Fill
  .$iif(%multi,$style(1)) Multi user : $iif(%multi,unset %multi,set %multi 1)
  .Length ( $+ $iif(%length isnum,$v1,3) $+ ) : set %length $?="Enter length of string"
}
alias -l effect return 6¤3.11·13´6`9·4.13¤ $+ 1 $1 7¤3.11·13´6`9·4.11¤1



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