gistfile1.vim

function! s:count_characters(bang) range abort
  let nchars = 0
  let lines  = a:bang ? range(1, line('$')) : range(a:firstline, a:lastline)

  for line in lines
    let nchars += strlen(getline(line))
  endfor

  if a:bang
    echomsg '>>> The file contains '. nchars .' characters.'
  else
    echomsg '>>> Lines '. a:firstline .' to '. a:lastline .' contain '. nchars .' characters.'
  endif
endfunction

command -range -bang -bar Count <line1>,<line2>call s:count_characters(<bang>0)