Being big helps, although a pit bull wouldn't hesitate to attack something many times its size.
The advice to avoid eye contact comes from dog trainers, a majority of which tend to be small females.
Roughly speaking, a dog facing you has five stances:
1) Tail up (possibly wagging), ears back: Playful and friendly.
2) Tail down, ears back: Submissive.
2a) Tail down, wagging, ears back: Submissive and afraid of you.
3) Tail down, ears up: Afraid, but willing to fight (this is really the situation where it is most likely to bite).
4) Tail up, ears back: Self-confident, hostile.
5) Tail up (possibly wagging), ears up: Self-confident, but neutral or friendly.
Of course there are intermediate stances, especially the hight of the tail is a basically continuous gauge of how inferior/Superior he feels.
Now, when confronted with a dog you can choose between a number of reactions:
Playful: Pant a little, lift your chin (but DON'T smile), clap your hands or slap you thighs. Call, - hey come on boy or similar.
Soothing, comradely (especially good with stance 3 and 5, above): Crouch, say -hellooo, good boy, slap your thighs.
Soothing, backing off: Don't stare, mumble - easy boy, back or crap away.
Threatening: Look him in the eyes, make low rumbling noises, and when (if) he vinces shout -SHOO! (NEVER lift your arms!)
What you do will depend on the situation, your self-confidence, size and type of dog etc.
....
One place i lived, there was a mini collie a few houses away. He used to hide behind the fence and when you came walking in your own thoughts he would suddenly bark loudly scaring the **** loose in you. One day, I was prepared, and the moment he barked, I turned, spread my arms and shouted BOOO! He fled, his tail between his legs, peeing all the way. After that, whenever I came by, he would stay silent.
Another time, I think I must have been 13 or so, a friend of my parents was visiting, bringing his rather fierce German Shepard. At some point during the evening, I took him for walkies (he was disciplined enough for that). Suddenly a fairly big dog came toward us, and since I weren't at that time too confident with dogs, I stopped, heart pounding. Then I remembered who I was with, so I took a step forward, instead. The German Shepard gave me a glance that VERY clearly said -"Yeah! Let's kick ass!" and advanced with me.
- And the other dog suddenly remembered an appointment he was late for somewhere about six block away

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Hans