How to force 4:3 in arena?

Find the file in the Arena directory which should be named something along the lines of arena.conf or dosbox_arena.conf and look for the line that says "aspect=false" and change it to "aspect=true" and save the file and load up the game.

Check out some of the videos in this playlist https://www.youtube.com/playlist?list=PL5T8bmLxd_T3YkOUep7KmcSMdvMZPtk5b and you can learn a bit more about DosBox config files and how to optimize their settings to fit your needs.

While the playlist is for specific games, the guidelines are the same for all dosbox games. Find the video and audio settings you like and adjust the configs on all your favorite dosbox games.

Thanks your link will be useful anyway but I found an other solution in french somewhere on internet and I will put It there in case It interest someone ;that just say to go to Nvidia configuration board then go in Display menu and chose adjust screen size and position then go in scale parameters and put the scale on graphic processor instead of Display in scale parameters.

Sorry about the syntax.