More information: when I change to full screen mode, my monitor claims that the resolution is 720x400 (it correctly reports 1280x1024 when not in full-screen). This is despite me changing the resolution to 1280x1024 in the config file. Also, I have now noticed that the bottom of the screen is also being cut off. I can move the position of the display using my monitor, but so much of the bottom section of the screen is being cut off that it won't let me move the screen up as much as necessary.

LCD's can be a pain, mostly because there's often something secretly rescaling the image somewhere along the process. Make sure there's nothing in your BIOS setting causing non-full resolutions to be automatically rescaled to fill the screen. Myself and another guy have been affected by that one -- difficult to discover. Also, make sure there's nothing in your driver settings to cause it. nVidia's WinXP driver has a section on LCD rescaling, and the default is to rescale to fill the screen. With nVidia, there are 4 possible settings.. and only one of them will prevent the rescaling. To sum up the problem: your gfx card driver, your LCD driver, your BIOS, and perhaps even the LCD itself all may be trying to secretly rescale the image, and together, they often succeed. It's best to discover all the sources and disable all of them.

Once that is solved, I recommend choosing 1280x1024, and also fullfixed=true. fullfixed=true will prevent the resolution from being changed when you switch to fullscreen from windowed mode. After all this, you can finally see the true effect of the dosbox settings and can begin tweaking things to your liking.

I had display problem with my monitor (which is some old crap), the image had an awful resolution.
I just had to change the output option (in the .conf) to openGL, and now, it works fine (all other output modes are still jerky)
I think you have to fix your LCD settings first, but this might help you -- or other people.

Thanks for the replies, but no luck. 🙁 I have searched through my BIOS, monitor, and gfx card driver settings for anything that looks like it might cause rescaling or affect the resolution in any way, and no luck. I also tried changing to openGL, and that made no difference.

No matter what I set the resolution to in the conf file, DOSBox will always go into 720x400 resolution. It's as if it's ignoring the resolution I set in the conf file altogether.

I don't see why the LCD would have to do any rescaling to begin with - I'm choosing a resolution of 1280x1024, which is the native resolution of my monitor. So augnober, why would a "setting causing non-full resolutions to be automatically rescaled" matter at all?

Thanks again for the replies... if anyone else has any suggestions, I'm all ears!

I've just made some progress, using gulikoza's advice from Video stretching for 4:3 resolutions (DOSBox/MAME Console). I set fullfixed to true and changed output to anything but "surface". (I had previously tried playing with both of these settings, but not at the same time.)

This makes a big difference - DOSBox now successfully uses 1280x1024 resolution instead of 720x400. However, for some reason this causes DOSBox to not use roughly an inch of screen at the top and bottom. So now, I'm getting pretty much the opposite problem: instead of text falling off the screen, not all of the screen is being used.

Even still, this is a big improvement, and means old DOS games are now playable. If anyone has any advice though on how to get it to the use the full screen, I'd certainly appreciate it.

I'm amazed at how DOSBox got sound to work straight out of the box! I was expecting to have to fiddle with IRQ settings and the like. Very impressive!

Actually the black bars are supposed to be there. 1280x1024 LCDs have a 5:4 aspect ratio, while CRTs usually have 4:3. Drawing small black bars displays correctly streched image...

I would just like to thank you, 'cause this solved my problem.
I had a similar, not identical problem: if used anything but OpenGL/OpenGlnb, fullsreen was 'shaky'. But if I used openGL, my CPU Usage went skyhigh, to 100%. So, if anybody else has the same problem: Fullfixed=true is the answer!