I made it work.

Code:
alias biggestfile {
  var %Count = 0, %LargestFileSize = 0, %FilesInFolder = $findfile($1-,*,0), %SubdirsInFolder = $finddir($1-,*,0), %TempSizeBuffer, %LargestFileName
  while (%FilesInFolder > %Count) {
    inc %Count
    %TempSizeBuffer  = $file($findfile($1-,*,%Count)).size
    if (%TempSizeBuffer > %LargestFileSize) {
      %LargestFileSize = %TempSizeBuffer
      %LargestFileName = $nopath($findfile($1-,*,%Count))
    }
  }
  return %LargestFileName ( $+ $bytes(%LargestFileSize,k3).suf $+ )
}


I'd still appreciate comments on my "final" version, if anything can be done better, etc.