Force Safari to open in fullscreen
You could save this script in ScriptEditor as an .app and name it something like "Safari Maximized.app".  Then every time you want to launch Safari in full screen mode, just launch your new app.  You may need to adjust the delay setting in the code.

This works for me in the latest version of Sierra

tell application "Safari" to activate
delay 1
tell application "System Events"
    perform action "AXPress" of button 2 of window 1 of application process "Safari"
end tell


Don't forget to allow access for your new app through gatekeeper