Your screen is a widescreen, the DOSBOX window not? But usually the black bars should on the left & right then...

Fiddling with the 'fullresolution' setting in 'DOSBox Preferences' (the config file for MAC OS) thus:

# fullresolution=1152X864
fullresolution=1280X768
# fullresolution=1152X
# fullresolution=640x480
# fullresolution=1152X864

using the first setting (commented out in the above), the black bar appears above and below the dosbox window. Trying all of the options above, leaving the line uncommented as above (fullresolution=1280X768) gives me a dosbox screen as long as the monitor with a black space to the right cutting off about 10% of the dosbox window. No amount of tinkering with the values for fullresolution beyond the ones I've listed has improved on this. It's acceptable, but, of course, I'd like to have it perfectly fill the monitor screen . . . Thanks for your interest.

Since I'm also on a Mac, namely PowerBook G4 Late 05, it's also a widescreen display. If I set a standard-sized display (i.e .640x480, 800x600,1024768), there are black boxes to the left and the right.

By standard-sized display sizes the Book offers two options: normal and stretched. If not stretched, the display results in black boxes at the border.

Maybe set to a stretched display before running DOSBOX?

Good suggestion, but my Mac Mini Display properties in Sys Pref has no option for 'standard' or 'stretched'; possibly that only applies for PowerBooks?