1. Right click the game in your Library then select Properties > Local Files > Browse.
2. Go to /Secret Agent/Secret Agent/Dosbox
3. Open dosbox.conf in any text editor (eg. Notepad, Wordpad, Notepad++, etc.)
4. Under [SDL], set "fullresolution" to match your desktop res set output=opengl

eg.

fullscreen=false
fulldouble=false
fullresolution=4096x2160
windowresolution=original
output=opengl
autolock=true
sensitivity=50
waitonerror=true
priority=higher,normal
mapperfile=mapper-0.74.map
usescancodes=true

5. Under [render] set aspect=true

eg.

frameskip=0
aspect=true
scaler=normal2x