The only way I have been able to figure out the size of a photo or movie in my Camera Roll is pretending I'm sending it as an email. I never send the email, but when you attach it to the email, it will show you the size. You can see the steps at this link: http://www.wikihow.com/Find-the-File-Size-of-an-IOS-Photo
You don't actually have to send the email. I just suggest these steps to find the size of a photo or video in the camera roll.
You can import the contents of the camera roll to your Mac using iPhoto or Image Capture: http://support.apple.com/kb/ht4083
Also, are you trying to get your iMovie project from your iPhone to the Mac for editing in iMovie or have you finished the project and want to save it on your Mac?
Very ingenious - thanks for the suggestion.
I've seen that "reduce message size" prompt in the past, but for some reason it's not showing up for me when I try it with a video now. Perhaps dynamically downsizing videos is not an option. Oh well.
This would be both for raw files to edit on the laptop as well as for finished projects. It's really to help me better manage space on my phone before I go to the "real" computer.
I'm glad that helped!
If you're just wanting to transfer the finished project, I recommend sending it to the camera roll and importing it with the link I listed above, but if you need to edit the movie in iMove on your Mac, you can use the steps in this article: http://support.apple.com/kb/HT4792