wow, I'll have to write an article about each of your questions...here goes

1. One becomes a witness by setting up a witness....@someguy123 has great tutorials on what you need to do as far as your computer. Then, one has to constantly campaign to become a Witness, as well as do work for the Steem ecosystem.
2. You would need to ask a Witness if they run full nodes, as I don't know.
3. The Steem blockchain runs in a decentralized manner, meaning through computers (nodes) all over the globe that are connected to each other. Steemit.com is different from the Steem blockchain. Steemit is hosted on AWS.
4. I don't know how big the Steem blockchain is currently, I think @someguy123 might know....and you'd need to ask @jesta, @good-karma or other Witnesses how many full nodes we have.