If by walking past each other you instantly 'have a thing for each other' then it shouldn't be that hard to say hi regardless of your height, because you already 'have a thing for each other' so there is no need to wonder whether or not she likes you. You're tall, she 'has a thing for you' and you may as well go for it because she quite possibly has a thing for tall guys!

I don't have to worry about this kind of situation because I'm only 5'8". Well, that and no girls 'have a thing for me' just by walking past me. Usually it requires a bit of work beyond that. If a girl has a thing for you just by walking past you, I wouldn't worry about height issues.