Give the gift of Drupal. All merchandise is 50% off through 2016.
Smaller functions doing a single task are easier to test and are more reusable.
If a function's entire source code doesn't fit within the screen of your editor, it's probably trying to do too many tasks and should be broken down into small functions.
On the other hand, if the entire source code of the function fits within the screen size, then it's more likely that everyone will be able to comprehend what that function is trying to achieve (and will be able to debug errors easily).