I also want to purchase a 3,5" screen and run Dosbox with my MOD called "Command & Conquer Retro". So i have Multiplayer with RPi3 and my Laptop on the go.
The developers of Dosbox have implemented ARM devices in their source code a few days ago so Dosbox now works optimized for ARM out of the box.
Have you tried 640x480 in config.txt and in the "dosbox.conf" settings "fullresolution=640x480" & "windowresolution=640x480" ?
And also try "output=surface" and "output=overlay".
Also try messing around with this --> https://www.raspberrypi.org/documentati ... t/video.md
You may consider installing a Desktop and try different settings with the "xrandr" command.
I also have a problem with Dosbox and Fullscreen, it does work when setting "output=overlay" but it is way slower than "output=surface".
Dosbox behaves different than on Windows and Linux for some reason. (maybe because of SDL1.2?)
Download Command & Conquer Retro MOD here (Windows,Linux,Online & ARMv7):
http://www.mediafire.com/file/2wmsboh6y ... ase_a0.zip
Download, unpack and run "CNCRetro_armv7_run.sh" - no install required. This is an alpha0 release - keep that in mind. Only tested on Raspbian Desktop.