As @Jorpho suggested, the display of text-modes in full-screen mode is affected by both the monitor and the driver.
In my case, I found that I needed to use my monitor's built-in control panel (the one that you access by pressing a button on the monitor frame). While DOSBox was displaying full-screen text - and ONLY while DOSBox was displaying full-screen text - I could use the controls on the monitor to choose an option to preserve aspect ratio. (This option was not available on my monitor's control panel when Windows was on screen.)
Also, I needed to play with various settings for fullscreenresolution= , output= and a few others in the same group. I'm not at the machine that has the wide-screen monitor right now, so I can't give details, but this should get anyone started who has the same problem.
One potential further problem: on a laptop, which doesn't have a built-in monitor control panel, this may require settings in the driver.