Approx 600-1200 MB for 1/2 or a full movie respectively...often Movies come in 2 parts. Trailers are shorter
If you right click on a file in the search pane you will see advanced>look up with bitzi. This will open your browser and ayou will see a page for the file and hopefully a rating and review of the quality or otherwise of the file.
You can contribute to the listing, If you download a video, good or bad, with no listing. indeed that is how it works. Hopefully you can see if a file is misslabeled (often porn), a poor recording etc.
Er ...... as for pirated copies, the only way to make sure you get the real thing is to got o the shop and buy it.
I hope this helps