So, here's some thoughts:
1)
If you go the Power Ranger Route (which wouldn't be a bad idea), keep in mind which ones would fit your height. With one rather obvious morphed/non-morphed change, most of the actors and actresses were roughly the same height as the person who would typically be in the suit with the helmet on. Hence, with the exception with Justin, there wasn't a whole lot of visible change. There were some that were more obvious than others, but, you'd be pretty much limited to the taller Rangers (which were typically male, I've noticed). If you want to do a Power Ranger costume, but not a Ranger, I think Goldar might be a decent choice, no idea how the height would be affected. Also, any of the Megazords would be good, and those you have a plethora to choose from as there seems to be typically two or three different formations per season. And yes, I have seen just about every single episode from 1993 to like '08 or '09. I have on my part disavowed RPM from the normal continuity. Sad, no?
2) Master Chief is a good route. All you need is a good under-suit (there is a thread on that, sort of) and a pair of good shoes, or my preference for me would be boots. My other suggestion if you stick with the Halo route is to try a Sanghelli (Elite) costume. It wouldn't be that hard to add on the minimum additional height (for you: minimum recommended is seven inches, for me, more like a foot and a half) in the feet and head/face.
Otherwise, the Predator and Alien would be a good choice. I would say the Guyver route, but the good guy characters are under six and a half feet with the Guyver on, and that includes the horn. Sorry. If you wanted to do a 'baddie' there's plenty you could do, and I would love to see someone attempt one.
For me, I'm going to want to be comfortable, so I know that with some of the extra space I have, I can put a few extra things to help me stay comfortable inside and to keep the thing on unless I want to eat or go to the bathroom. I'm not exactly buff, so it helps...
