See above. The manual recommends 1GB free space.
It has to vary between disks. I got "Transformers" on Blu-Ray last night, and was fooling around with the downloadable stuff. Based on the download speed, they couldn't have been more than a meg or two. I read somewhere (not sure if was about this movie or not) that at some point you'll be able to download trailers for new movies (oh boy! advertisments!) that would have to be tens or hundreds of megs at least.