I'd say that a small majority of guys don't care, while a sizable minority of guys would care if the girl is taller than he is. Not sure how tall you are, but on average, men are 4-5 inches taller than women, so this shouldn't be that difficult to overcome. Either you find a taller guy than you, or one of the guys who doesn't care if you're taller than he is. That probably excludes less than 20% of guys.
Not at all. I'd feel just the same as always no matter the height of the girl. I'd still be the same person regardless of her height. And besides, Mario doesn't seem to mind that Peach is taller than him by a great deal. I know he's just a fictional character, but still, it shows that a guy can be a masculine man like our heroic plumber and have a taller girlfriend.
And to be honest, I've seen a lot of girls between 5'11" and 6'8" with an amazing kind of figure that you only see on girls that tall.