Master of Orion 2 and likely other games.
I have an Acer Vero 514 and I have activated the linux which is Debian 10 (Buster)
I have installed the Heroic Launcher.
It connected to GOG and I downloaded Master of Orion 2
The game starts and runs in DOsBox but only in a small window.
How to I get full screen?

Alt+Enter. Failing that, you might have to do some jiggery pokery with the configuration to see if a different graphical overlay will do.

Failing that, upgrade to either a source port or Dosbox Staging where possible (regardless), because why deal with a version of Dosbox that's still reliant on such old tech?

I think you'll find a .conf file in your installation you can edit with a text editor. Change the full screen property to true instead of false.

If you can choose the rendering option in the conf file, going to SDL may give you full screen, as SDL is a simple display layer, not really intended to mix with the likes of XWindows. (Though i'm sure DosBox will have a XWindows, GTK or other addons to support the current windowing method too)

Though more than likely doing fullscreen=true will probably be what you want as mentioned earlier.

https://www.dosbox.com/wiki/Dosbox.conf%C2%A0#Linux