What are you using to record the video with? Is it SD or HD? If it's HD and your sending it to iDVD, then I wouldn't throw away the source files because they contain 4x better resolution. And someday, you might like to look at those videos on a nice, big 60 inch TV, and not have them look all blurry.
50i is NOT the same thing as 1080p. I would record in 1080p since the quality is better (though admittedly, the real world difference is often very small).
But regardless, if space is a concern, I would edit my project, Share as a file with the highest quality setting, and then trash the source when you need the space.
I would NOT use the DVDs you've created as your final record. Like I mentioned before, your source material will look great on a big HDTV. Your DVDs won't look anywhere near as good.
1. Usually when i have made some video recordings I plug my kamera into my mac and import the video(s) into iMovie. (My iMovie Libary is locaded on an extern harddrive).
Nothing wrong here, but as I said, I would record in 1080p for maximum quality.
3. When I'm done with the project I choosing share in iMovie and export my project to a place on my mac. For some reason the file always become many times larger than the original videofiles even though I didn't ad that many features to my porject?
My first thought this is because you are recording at 1080i, but outputing from iMovie as 1080p. But your step 5 paragraph calls this into question.
5. Than i Open iDVD and starts a new project where I'm adding the converted videofile from iMovie.
I'm really curious as to what settings you're Sharing the video at from iMovie. I thought iDVD required 480p from iMovie, but I could be wrong.
But what I really want to stress here is that the DVD that iDVD creates will be significantly lower quality than what you're recording with your video camera. You may need a DVD now, but someday, you or your kids will have a big HDTV, and the 1080p version of your video will look MUCH better than the 480p DVDs you're creating now.
So, the step I would add would be to Share two versions of your projects from iMovie: one you'll use for your DVD, and a 1080p version that you keep on your hard drive for the future when you have a nice big TV to view it on.
BUT that do I do with the above files I've made? What should I safe for later now that I have burned my project?
I know that I would like to keed a digital version of the finish product - but how?. I wan't as well to safe the original files in iMovie if the future I should need to make other kinds of projects.? All this seems to cloudy to me! Please help.
As I mentioned before, I would Share the project as 1080p, save the file to a drive, and then delete the source material when you need the space. If you have the space, just keep the source files in iMovie forever.