The only limit that I know of is the size of your hard drive. The above poster is partially correct, iMovie does force you to limit your clips to 2GB. However, it will will split your video into clips for you while you're importing. You can string together as many clips as you want to make a movie as far as I know. The only time the clip size issue comes into play is if you're using clips imported from some other application. Some movie editing apps support 4GB clips. If you don't trim those down iMovie will not be able to use them.