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also ran in Debian & Ubuntu GNU/Linux the same way, but it's been decade+ since I've wanted to run them as they're outdated (with many no longer available, as they were re-written/replaced and it's simpler being able to use a WM than controlling everything from command line).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).

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

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

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.

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.