Depending on the site and the stream quality movies can vary in size. For instance, Netflix allows for three different quality settings consuming different amounts of bandwidth :
"Users can choose from three quality settings by visiting the “Your Account” page on Netflix’s website and looking for the “Manage Video Quality” link. “Good quality” consumes up to 0.3 GB per hour, “Better quality” burns up to 0.7 GB per hour, and “Best quality” consumes up to 1 GB per hour for standard definition or 2.3 GB per hour for HD. The settings apparently apply to computers and televisions."
Hulu does not publish how much bandwidth their movies consume but representitives have said 700kb/s
"Our videos normally stream at 700 kbps, depending on your bandwidth,
so a twenty minute program would average around 100 MB. "
Sites like YouTube and Vimeo usually will provide lower quality video as the viewable size is smaller then that of your TV.