If you have a big widescreen mon it usually isn't a good idea to go full screen, then you can edit the dosbox conf file to set the window bigger.
Something as:
[SDL]
fullscreen=false
fulldouble=false
fullresolution=original
windowresolution=1024x768
output=ddraw
And before you ask:
<- click
And read the dosbox manual, there are a lot of stuffs you can do.