you can find the official Dosbox forum over at VOGONS: https://www.vogons.org/viewforum.php?f=53
People are very active on the forum, and you’ll find lots of answers there.
For starters though, and not to have you leave here empty handed: you should set Dosbox to fullscreen, and experiment with the fullscreenresolution option (https://www.dosbox.com/wiki/Dosbox.conf#.5Bsdl.5D).
For example, fullscreenresolution=640×400
Also, if you don’t want the original pixelated image, you might want to try the different scaling options. That way, you can achieve a smooth image even in “full” fullscreen mode.