UPDATE: Thanks to Rick's comment this solution is now perfect for me!
The below solution isn't perfect, but it's good enough for now. I would prefer this solution to use 40 char width for the netrw buffer instead of 20% of window width. If anyone knows how I'd greatly appreciate it.
On ctrl-n toggle Vex and limit it's size to 20% of the window width. I borrowed (ie stole) from answers to Toggle explorer window.
nmap <C-n> :call VToggleNetrw()<CR>
let g:netrw_winsize=20 " percentage of window width
let i = bufnr("$")
let wasOpen = 0
while (i >= 1)
if (getbufvar(i, "&filetype") == "netrw")
silent exe "bwipeout " . i
let wasOpen = 1
" Added from Rich's comment. Resize netrw to 40 chars wide
:vertical resize 40