Yep I found that out last night - I reinstalled the original sdl package and started prboom and wonder of wonders it started in a window, not full screen but bigger. I took a peek at the output of \'ps\', then tried starting it again and it went back to being little (but in a window). I couldn\'t remember the command line switches which it had previously but I\'d guess that they are screen_width, etc.
Thanks,
Si