Is it possible to have Full Screen with 4:3 ration on 16:9 monitor?
Where can I configure this?

Use scaling options of your GPU control panel/center.

Mark the "Keep aspectratio" in the advanced section of the graphics setup program.

This only works if the game is played in windowed mode. When full screen is checked the 4:3 game is stretched.
If I have to play in windowed mode, is there a way to increase the window size to more than normal3x?

Edit the .conf file and set:

[sdl]
fullscreen = true
fullresolution = desktop
windowsresolution = anything you like
output = anything you like except surface

[render]
aspect = true

http://www.dosbox.com/wiki/Dosbox.conf

Thanks a lot! It works.