Put this in your ~/.config/openbox/lubuntu-rc.xml in between <applications> tags:

<application class="Firefox" name="Navigator">
    <fullscreen>yes</fullscreen>
</application>

To get specific class or name for your applications, use this:

obxprop | grep '^_OB_APP'

and then click with your cursor inside the app window.

Useful website: http://openbox.org/wiki/Help:Configuration

What's your setup? Do you actually have 2 separate X displays, or are you using Xinerama to combine the two?

If you're using 2 displays, they should have their own -display or DISPLAY env variable. You can use that to execute a new app.

$ echo $DISPLAY
:0.0
$ xterm -display :0.0

Open a shell in the other display to get your other display env variable.

There's also -geometry, in the format of [Wth,Ht+X+Y]

$ xterm -geometry 132x80+500+0

When Xinerama is enabled, your other display is usually a +X+Y offset.

Of course, this does require the app to process command line arguments. It then passes those off to X when initialising the window. For xterm, width and height are units of text characters, where X/Y are screen pixels.

Yes.

You haven't provided any details of your GUI application; and importantly I think what stacks it uses.

Is it motif? simple xorg?, using GTK2/3? Qt5? etc as most applications require specific stacks that need to run - but if it's a simple X11 GUI app that doesn't use stacks I believe it's a yes. However very few GUI apps remain today that don't use GUI libraries/toolkits.

I've opened GUI apps on unix/sun-OS boxes where the size was defined in the command that opened the window, and full-screen was possible, however they weren't modern apps & didn't use modern toolkits. Many of those programs also ran in Debian & Ubuntu GNU/Linux the same way, but it's been decade+ since I've wanted to run them as they're outdated (with many no longer available, as they were re-written/replaced and it's simpler being able to use a WM than controlling everything from command line).

You however gave no specifics; but it is possible for some applications (just not worth it in my experience; we no longer run machines with that limited resources).

Based on @dirkt's comment I went and had a look at wmctrl, which is a command line tool that lets you set these yourself. If you'd like to do things like toggle fullscreen for whatever programs you like from a shell script or any other custom place, this is ideal for that.

Here's an example that toggles fullscreen for the currently active window:

wmctrl -r ':ACTIVE:' -b toggle,fullscreen

For more details on what you can do with this see the man page and for some example see this tutorial.

Some programs in Lubuntu remember the size when they were closed. Some do not. Lubuntu users can open some or all programs maximized by editing ~/.config/openbox/lubuntu-rc.xml. A very detailed how to can be found at the Community Documentation site

https://help.ubuntu.com/community/Lubuntu/Windows#Launching_Windows_Maximized

I think you are looking for kiosk mode, you can achieve this by various kiosk based linux based iso distribution like http://sanickiosk.wikidot.com/ (Sanickiosk) and WebKiosk (http://www.binaryemotions.com/).

Even you can customize ubuntu to run only firefox in full screen mode (http://www.instructables.com/id/Setting-Up-Ubuntu-as-a-Kiosk-Web-Appliance/?ALLSTEPS).

Thanks & Regards,
Alok Thaker

What is the problem with F11?

There are different domains where you can decide if your application runs in fullscreen mode or not.

1) The "X-Borders" are drawn by your window manager. An application running on your X-Server without a window manager won't have any borders. If theres a fullscreen mode for the application depends on the window manager itself.

For example in my i3 you can use the key combination ALT + F to switch to fullscreen mode.

2) The used graphic driver behind your library.

GRX supports different drivers and you can set them with the following function:

     int GrSetDriver(char *drvspec);

The SDL driver supports a windowed mode and a fullscreen mode. So

sdl::fs 

should set your application in fullscreen mode.

See the manual for more information:

http://grx.gnu.de/grx248um.html#Setting-the-graphics-driver