Males will usually be in the 3.5-4 foot range, females may get up to 4.5-5ft. They are NOT big in the sense that you may be thinking of as big. They are bulky(think slightly thinner than a soda can when adult), but they do not get big at all.

Your tank is going to be harder than a snowman on a cold day to heat because of all of the air space. The Ball Python is never going to make any use of that space, not to mention that it is entirely too big for a baby ball. Such a vast amount of open space is very stressful on such a small snake, and he/she probably won't feel secure enough to eat until the situation is remedied. You may want to go to WalMart or Target and get a smallish, locking underbed box such as a Sterilite/Rubbermaid/Iris(16-24quart). It will make your snake feel very secure, they are easy to clean and heat/keep humidity in, and are easy to replace when the snake outgrows them. Pick one that has more floor space than height(think 6-7" in height). Put 2 -3 lines of small holes using a soldering iron($3 at dollar store or $10 at WalMart) along the top edges of the sides. Do not put holes in the lid.

You also need one or two more hides. You can get clay pot SAUCERS at WalMart. Saucers are what the planter pot sits in and it catches water. You knock small entrance holes out of the sides with a hammer and turn them upside down. They make excellent hides as they are low to the ground, small and dark. You want one in the cool side and one on the warm side.

As for temperatures, you will want to measure them accurately with a DIGITAL thermometer/hygrometer(measures humidity). You can get a good model by the name of Acu-Rite at WalMart for around $12-15. When you get it, place the probe in the warm side hide and then place the unit itself flat on the floor of the cool side. This will measure the hide temps on the warm side and the floor temps on the cool side as well as the relative humidity of the enclosure. Accurately measuring your temps are EXTREMELY IMPORTANT in the care of Ball Pythons. They can be very specific on what their temps and humidity are, and any fluctuations won't be a good thing on their part.

Speaking of temps and humidity, temperatures should be between 92-94 on the warm side and 82-84 on the cool side. Ambient(air) temps should be in the low to mid 80's. No temperature should ever fall below 80, and if it does you are asking for a respiratory infection for your snake. Humidity should be between 50-60%. You can up it to around 70%, or if you decide to house the snake in a plastic underbed box, then you won't have to worry about it.

UNTIL you get everything situated regarding housing, temperatures/humidity and proper hide spots, then do not feed the snake or handle it. After everything is up to par, then wait a complete week for the snake to get settled in the new surroundings, and then attempt to feed, in the enclosure and at night. After the snake has successfully fed 2-3 times, and you have waited AT LEAST 48 hours after the last meal, you can begin short handling periods of about 5 minutes per day for a few days of the week.

In my experience, you really need to get the housing and heating situation under control. That size of tank is really not going to be the best environment for a small ball python. Go with either a 10gallon(long) sized tank, a 20 long, or better yet, a locking, small, underbed box that is around 16-24 quarts. Floor area is much more important than air space. Besides, the plastic housing is MUCH easier to heat and the slightly opaque sides help the snake feel secure. You will have a much better snake-keeping experience, as well as less hassle on getting your new snake to eat, if you decide to go with that housing option.

Hope this has helped a bit, and if you decide to not keep the Ball, another good beginner snake that doesn't get very large is a Cornsnake. They may get around 4-5 ft, but that 4-5 ft is extremely slender. They don't stress as easily and are very hardy. Whatever you decide, we'll help you get everything straightened out

Good luck!