Check Dosbox Guides.
Same method as speeding dosbox up.

Edit: oh, it's you the cga-guy 😉 I would never expect you asking such a trivial question 😉

Thanks, a very simple solution. Sometimes I'm just not on my game.

I do have one error to report with the scaler option. While the 320x200, 640x350 and 720x348 modes show up properly, the 620x200 modes don't. In this mode, used for example by Infocom v6 titles, essentially I should get a very rectangular window. While the mode appears properly in Dosbox screenshots, in the Dosbox window the resolution is line doubled to 400 lines. Fix?

I think it should be like that. Have you checked it on an old cga card and old cga monitor?

It should look Squarish, not Rectangular on a CGA or EGA monitor. After all, the text modes don't fill only half the screen. The CGA monitor really is supposed to show 200 lines, the EGA monitor 200 or 350 lines. Those lines would fill the screen and the pixels in the high resolution (640) modes would be half as wide as those in the low resolution (320) modes. However, in a window, I would like to see only 200 lines, not 400 lines with doublescan, because I can only run Dosbox in a window at full speed when the window resolution is at its lowest possible.