nerdtree_plugin/toggle_width

toggle_width.vim

" put this script in ~/.vim/nerdtree_plugin/

call NERDTreeAddKeyMap({
      \ 'key': 'w',
      \ 'callback': 'NERDTreeToggleWidth',
      \ 'quickhelpText': 'Toggle window width' })

function! NERDTreeToggleWidth()
  let l:maxi = 0
  for l:z in range(1, line("$"))
    let l:aktlen = len(getline(l:z))
    if l:aktlen > l:maxi
      let l:maxi = l:aktlen
    endif
  endfor
  exe "vertical resize " . (l:maxi == winwidth(".") ? g:NERDTreeWinSize : l:maxi)
endfunction