I'd say that's a fairly conservative estimate. From a quick look through Anytime plus on my box, a standard-def film is about 1gb, high def can be 3gb or over, one of the HD films I looked at was nearly 3.7gb!
To check the file size, just highlight the movie in the menu then press the i button on the remote. You'll then see the duration, programme size and when it expires from Anytime.
Downloaded stuff goes into 'your' section of the box. Stuff on the Anytime Showcase is already on the Sky reserved section.
You can't schedule downloads for a certain time, as far as I know. But it's easy enough to just start a movie or movies downloading when you head to bed.
It can be a bit weird, I was trying to get the 2nd episode of Game of Thrones the other night, it initially said the HD version (51 minutes) was about 2.3GB, however having downloaded 1% it was then saying it was 11.5GB! As that would have taken me well over my monthly d'l limit I cancelled it and got the SD version, which was 680MB.