The Official Rules Of Golf - Tee Boxes
Each hole on the golf course will have it least 3 tee boxes positioned at varying distances from the putting green with the closest normally being marked with a red tee marker.
Tee boxes will be defined by markers on each side which are typically placed 10 yards apart and a tee box is generally about 9 feet, or 2 club lengths, deep.
Although this may seem like a large area this is done deliberately to allow you as much variety as possible to set up for the particular shot you wish to make and you can tee up your ball anywhere within this 10 yard by 9 feet box. It is also interesting to note that according to the official rules of golf you do not need to stand within the box. In other words, it is okay for your feet to be outside the box as long as your ball is within the box.