Steam installation = DosBox? or did you already patch to TombATI? (Which I really recommend doing, if you haven't already)
If you have patched to TombATI, For changing the resolution you would need to go to the "patches" folder in your game directory and find either "dosbox.ini" or "tombati.ini", open it and you can change the resolution from there.
For windowed/fullscreen, open the "glrage.txt" in the main game directory.
Hope I helped.