Ve složce s Dosboxem je mimo jiné manuál. Jmenuje se Readme.txt, v sekci FAQ je odpověď...

No dobře, dám to sem:

Q: How do I change to fullscreen?
A: Press alt-enter. Alternatively: Edit the configuration file of DOSBox and
change the option fullscreen=false to fullscreen=true. If fullscreen looks
wrong in your opinion: Play with the option fullresolution in the
configuration file of DOSBox. To get back from fullscreen mode:
Press alt-enter again.