you have to modify the file "dosbox.conf", which is the config file for dosbox. there should be a shortcut to it in your start menu (assuming you're running windows). if there isn't look for it in your dosbox directory.
open it.

assuming you're running the latest version of dosbox, these are the lines that are of interest:

fullscreen= fullresolution= output= aspect= scaler= […]
Show full quote

fullscreen=
fullresolution=
output=
aspect=
scaler=

fullscreen : you can set this to "true" or "false". when set to true dosbox will start in full screen. you can switch between fullscreen and windowed by pressing alt+return any time.

fullresolution: enter the resolution you want dosbox to run at (i.e. 640x480)
you can also set this to "original" which will attempt to run the games at their original resolution. generally a good idea but you will only get a refresh rate of 75hz. i recommend setting it to "desktop". that will run dosbox at your desktop resolution.

output: different options of video output. "surface" produces a very clear picture but is slightly slower than "overlay" which i would recommend you to use (assuming you're not running the games at their original resolution in which case i would use "surface")

aspect: "true" or "false". true enables aspect ratio correction. enable this when not runnig games at original resolution.

scaler: there are lots of different kinds of scalers. try them out and see what you like. again when using original resolution don't use scalers (set to "none")

here is my recommended setup:

fullscreen=true
fullresolution=desktop
output=overlay
aspect=true
scaler=normal3x

these settings scale the original game to your desktop resolution enhancing it by 3 which will help reduce strange looking text and stuff due to the different pixel aspect ratio...

hope this helps

Thank you. World of Xeen still doesn't work but I think the problems comes from the game itself. I'll download another one.

Anyway, it's fullscreen and good quality. Thanks

did you try to adjust the cycles in your dosbox.conf?

depending on your hardware i would suggest you try the following settings:

frameskip=0
core=dynamic
cycles=max

if this doesn't work or the game runs too fast try setting the cycles manually like this

cycles=5000

you can adjust the cycles of dosbox by pressing ctrl+f12 to speed up and ctrl+f11 to slow down. dosbox displays the amount of cycles on the top of the window in windowed mode.

you can tell dosbox how big a step to take with each press of these combinations with:
"cycleup=" and "cycledown="

alternatively you can also try core=normal or core=auto