Adjust the vertical size controls on the front of your monitor? Set a different video mode for full-screen?

If all else fails, look at the "fullfixed" and "fullresolution" options in dosbox.conf

I've changed all of the Dos Box .conf resolution settings and it still won't work, and I've already adjusted my monitor, and it has the same effect.

Then you should tried tinkering with output= parameter if you haven't already done so.

Start, by using output=surface, but you should test all available modes, i.e, surface, overlay, opengl, etc...

PS - Are you sure you are properly selecting the best resolution?
Could you post here some excerpt from your DOSBOX.CONF?

I was having similar issues as above, this worked for me:
fullfixed=true
fullresolution=640x480
now only have about 1 inch of blank space on the left side, which is MUCH better than having most of the screen out of view.
If I use 800x600 for fullresolution, it's more centered, but has maybe 1 or 2 inches of black around the sides

Check your scaler= setting too.