How to change size of NERDTree window

The NERDTree window width is set to 31 by default. You can check the current value of the width using:

:let g:NERDTreeWinSize

To set the window size to some other value:

:let g:NERDTreeWinSize=20

To make this change permanent, put this line in your vimrc file:

let g:NERDTreeWinSize=20

Tried with: Vim 7.4 and Ubuntu 14.04