Linux shell scripting is a very powerful tool which I had a hard time learning and I am still a padawan.
The script you found uses conditions to decide if a certain part should be executed.
The first two commands unset named environment variables.
The third starts an xfce4 session - but because you have not added the continuation marker - the script will stop execution until the xfce4 session ends - then it will continue with the remaining commands
exec is a keyword which turns over control to the command - which in turn will stop execution of the script.
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
The above line verifies it the file exist and if it does - the shell turns over control to xstartup script
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
This line will execute xrdb if the file .Xresources is found in the user’s home.
To learn what xrdb is capable of use the xrdb(1) — Arch manual pages
man xrdb
As you will see - you can add your preferred width and height to the xrdb command - like this
[ -r $HOME/.Xresources ] && xrdb -height=900 -width=1300 $HOME/.Xresources
Then it executes the command x-window-manager and releases the script.
x-window-manager is an unknown command - at least on my system …