1) How big do ball pythons generally get. Everything I saw said 3-5 feet but some look very large in pictures. Total weight?

3-5 feet is fairly accurate. Althoug I have seen some get to 5.5 feet. Most females will max at about 3000 grams (about 6.5 ponds) although I have seen some go to 7000 grams (15.4 pounds). I have heard of 9000 grams, but that is an extreme rarity.

2) Would a fully grown ball python fit in a 20 gallon aquarium?

No a 20 gallon would be too small for an adult female. If you had a male, you MIGHT be able to make it work, but I would suggest a 40 gallon breeder for an adult ball.

3) How often do they "go to the bathroom"

That depends on how you feed them. However, most of mine that eat well go about every 14 days. Very low maintainance snakes after you have their environment set up properly.

4) Can humans get the mites or parasites on them that some snakes carry?

No. mites are host specific, and only feed on reptiles. They can get on you, but they just crawl around. They will get off of you, because you have nothing they need. With that said, if you practice clean husbandry, you should never see a mite, and if you have a fecal done on your snake when you get it and get a clean bill of health, than you should have nothing to worry about.

5) Does a bite from a full grown ball python hurt?

It can, but most of my balls never bite. Also remember that most of the time the reason it hurts is because you pull away when they bite. If you leave your hand still when they strike, it usually is over before you kno what happened.

6) How much can you handle them without stressing them out?

It is not a dog or cat, so they don't like to sit in your lap for hours at a time, mostly because our body temperature is 98.7 degrees, and that is too warm for them. However, as long as you avoid handling for 48 hours after a meal, you can handle your snake for short periods of time everyday.

7) How tame do these snakes actually get?

Ball pythons are known to be the most docile snakes in the world. With that said, I don't really think they tame down any, but they are not prone to bite unless they are in feeding mode or are threatened.

I hope some of that helped, and I'm sure that some other members will expound on what I said.