DosBox can be switched between windowed and fullscreen modes by pressing [ALT]+[ENTER].
You might also want to check out
the official DosBox FAQ, and particularly
this part of it. It is quite helpful for users unfamiliar with DosBox.
And don't forget the readme file. It's got all the keyboard shortcuts listed.
You can configure the starting values of frameskip, cycles and so on in
dosbox.conf.