mel scripts can easily grow to fairly large sizes. As a general rule, i would advise keeping all of your scripts to less than 1000 lines. Above that size and you will soon get lost trying to perform the simplest task.
Ideally then, we need a way to break our scripts into seperate smaller ones. In addition to simplifying our scripts, this process also promotes much better re-use of code since each small script may be used by a number of other more high level scripts.