[DSpider]

I think the .epub file could, theoretically, take up several GB (well, probably not since it's a zip archive after all and text compresses very well), but the content itself should be split into different .xhtml files either by title, chapter or if it's something really really long (doubtful), you could segment it as Part I, II, etc. or simply just cut it at a period (.) and it will resume the next page starting at the top.