Code:
alias bignick {
  var %i = 2, %bignick = $nick($1,1)
  while $nick($1,%i) {
    if $len($nick($1,%i)) > $len(%bignick) { %bignick = $nick($1,%i) }
    inc %i
  }
  return $iif($prop == len,$len(%bignick),%bignick)
}


$bignick(#channel) - returns the longest nickname.
$bignick(#channel).len - returns the length of the longest nickname.

New username: hixxy