EPUBs sent to Apple for publishing in the Mac App Store have a 2GB maximum size. Apple does not state the maximum video size. I just dragged and dropped an 800MB (.mp4) movie into Pages, let the page of text flow around it, and exported to EPUB using the Fixed layout option.
When I opened it in iBooks, there was a delay loading the video with a white square in the middle of the black movie window. Eventually, the movie loaded with play controls and began playing with audio, when I clicked the play button.
Unless you are the creator of the video, its inclusion in an EPUB without permission may come back to bite you from a copyright infringement issue, and if destined for the iBooks Store, you will have to prove to Apple that you are the copyright holder, or have written permission by the video owner to use it in the EPUB.