The most popular length is 25-foot, although 16-foot and 30-foot are popular size, and includes both standard (imperial) and metric markings. The unit you choose should have a locking mechanism that will allow you to pull out the length of tape you need and then lock the unit so that it won't retract until unlocked.
Also notice that the hook at the end of the tape should be slightly loose on its rivets, which is by design. This allows the woodworker to accurately take both inside measurements (when measuring the distance between two walls, for instance) and outside measurements (e.g., when determining a length of stock to be cut). The amount of looseness should be the same as the thickness of the hook.