Is there a way to make the screen bigger? I have a black box around the game

It is just a little picture with a black border because your LCD monitor doesn't scale it. You have 3 ways to fix that:
1) change the settings of your monitor (look in the manual for that)
2) in the dosboxBlakeAOG.conf (or dosboxBlakePS.conf if it's the second game) in the game directory change the "fullresolution=original" line to "fullresolution=AxB" where AxB is the resolution of your monitor (1280x1024 for example). You'll perhaps want to change the scaler DOSBox uses (the scaler= line a bit farther down), but you'll have to experiment around with that if you don't like the look the standard scaler produces. There is no real agreement over the "right" scaler, it's dependant on personal taste. And perhaps you'll want to change output= to opengl or direct3d for hardware acceleration.
3) Look up if you can set your graphics driver to scale everything to the resolution of your monitor.

1 and 3 are a generic solution that will fix that problem for every game, but could look worse than number 2 or have other problems. 2 works for every DOSBox game but could look bad or have problems with slowdown for big resolutions or slow hardware. Experiment around a bit. Oh and you shouldn't change the resolution to something with an aspect ration other than 4:3, i'll look weird.