Probably the best roll bar out there is the Elda Engineering unit. You can see pics and reviews here -
http://www.s2ki.com/forums/index.php?showtopic=147070
I'm sure there's plenty of people your height that have found ways to get some extra room in the car. Your best bet will be to ask over at
www.s2ki.com since it's an s2000 specific msgboard.