Yes, your movie is too long. Divide it up into 5 30 minute movies. You can do this by selecting your movie in the Project Library View, and clicking FILE/DUPLICATE PROJECT 5 times. Then for project 1, delete everything but the first 30 minutes. For project 2, delete everything but the 2nd 30 minutes, etc. If 30 minutes is still too long, you can shorten it again with this method.
If you need to make a theatrical length movie (e.g. 2 1/2 hours), I suggest Final Cut Pro X. It will make movies this long.
Final Cut Pro X can import iMovie Projects, so if you want, you can create your movie in Final Cut Pro X.
The reason iMovie is not the right tool for you is that it was designed for amateur home movies. Technically, iMovie will render the entire movie in a single pass. This means that the longer your movie, the more powerful computer, memory, and disks you will need, but ultimately there is a limit to what can be accomplished in a single pass. However, Final Cut Pro X can do multiple renders. It will render as you go, so that at the end, when you make your 2.5 hour movie, it can just string together sequences which have already been rendered.