to ignore "node_modules" sync in frob box , just create a symlink to a folder out of dropbox
my app is in d:\dropbox\myapp
my external folder is d:\node_modules\myapp
so i just need to run this command in an elevated "cmd" terminal : (dropbox will ignore dir junction symlinks automatically)
mklink /J Link Target
mklink /J d:\dropbox\myapp\node_modules d:\node_modules\myapp