Alright so I'm 6'6" I'm a pretty big guy (not necessarily fat although I need to go to the gym more) I tend to intimidate people as a general rule. Especially in arguments (I don't get too aggressive and even then I start to back away). I'm very self concise of my size so I've very rarely gotten into fights with other guys and never have touched a girl out of anger. I have apparently intimidated girls just on my size (again not fat pretty average). I'm not bad looking and have had very hot girls want to be w/me (party 24/7 girls just aren't my type anymore though). So why do I intimidate the average girl just on 1st impressions. They have never met me and usually they have never heard of me. Sometimes I feel like a circus freak.
I think in todays society we are being conditioned to see men as potential attackers so the bigger the guy, the less chance we have of being able to defend ourselves. I know this sounds stupid but we think about it. And its even worse for girls who are victims of rape or physical violence. Its not that women don't trust you as a person, but we don't trust men. And not many guys are 6'6" so its a little unexpected, not freaky, just different.
Now on the flip side I LOVE tall men. I don't think I've dated a guy under 6'0 and I'm 5'2. It all depends on the girl and how comfortable she is.
Yeah, a guy as big as you intimidates me a bit, but I've got at least three friends who love guys that tall. I think it's not too different from any other physical preference, and you're probably better off being tall than short.
You mentioned party girls aren't your type ANYMORE, which I am assuming they once were. This is probably where your problem lies and I bet you are using the same style of approach (consciously or sub-consciously) when you try and meet your "average girls"
Next time you want to approach a girl, think about the energy level you are approaching with. You are a really tall guy, and you keep mentioning you aren't fat, but by this you aren't a stick either so I can see where you are coming from when your overall size intimidates girls so much. Try keeping your distance when you approach girls, and use very low energy openers, emphasizing that you may be a big, but you are very gentle when it comes to girls.