I am still using the trial. Having difficulty figuring out how to do something simple.
I draw a mask rectangle and I want to trim the movie to it. I can't seem to find a command to trim it.
There is no such thing. Your mask is your trim. It does not change the physical size, it only hides unwanted pixels. This is just how it works. If you are speaking about adapting all the comp to your footage, you can however still do it. Enable the limited region at the bottom of the comp viewer, then position the rectangle to encompass your movie. Then you can choose Crop to Region of Interest in the Composition menu.
Like Mylenium said, After Effects doesn't work like that. Masks and everything else in AE is non-destructive, which IMO is a very very good thing.
Anyway, if you wanted to crop a large image like you described, one way would be to make a small composition for that image and the put that small comp inside your final comp, that way the image will be cropped to the size of the smaller comp.