It launched by default as a small dos box in the middle of the screen, pretty bad/annoying to play with. After some searching there’s a config file you can edit, whenever your install is/dos → dosbox.conf
the top sections has
[sdl]
fullscreen=false
fullresolution=desktop
output=openglnb
Change the fullscreen=false to fullscreen=true and it will boot in fullscreen.
Would be nice to just be an option somewhere as it’s not intuitive at all to find out how to do this 