For the 700p, a 4GB card will work. (For the 650, the limit is 2GB unless you install a FAT32 driver.) I don't know file sizes, but the files will be MPEG4, I think, not to mention reduced pixel count, so you should be able to get quite a few hours out of a 4GB card.
I expect it to be similar to iPod video files, which are often MPEG4. Looking at some files I have, a 1hr8m "This Week in Tech" MPEG4 video is 151MB. You can extrapolate from there.