If you are one of the lucky ones who has the creative knack of expanding out a script
as you go then the actual length of the script is not all that important. Certainly quite a few filmmakers have taken a short script and made it into an extensive feature film. Experience definitely helps a filmmaker fine tune a script and the longer you've been doing it, the easier it is to flesh out a script. The main ingredient to succeeding is having a good script to begin with that is creative and has plenty of detail. This can help you get a grasp of how long your feature film length will be and make the whole creation process more enjoyable.