You need QT Pro to access properties, but even this won't help you if you compress your film from iMovie to Sorenson, for instance. Since iMovie exports with "High quality" off, you'll get the the lower quality in compression and the high quality setting will have no effect.
This is a serious bug in iMovie (and/or QT 5). The workaround requires QT pro ($30) and a bunch of additional steps (exporting in full quality, checking "High quality" (and probably "Single field)in QuickTime Pro and finally exporting with your desired compression). I'm not even entirely sure that it works (I don't use iMovie anymore) and even if you manage to export in full quality, you might have problems with interlacing (QT 5 seems reluctant to deinterlace)...
If you're ready to move up from iMovie, try
. It's free and a lot more capable than iMovie (albeit harder to use). It also includes Media Cleaner EZ that handles compression.
It's, as I said, not nearly as simple as imovie, but you can do a lot more so it might be worth trying. Beware that there are incompatibilities with iMovie, so read the Readme carefully. If you decide to go back to iMovie you can keep Media Cleaner and use it instead of QT Pro for compression. Might save you the $30.