Emacs versions before 19.29 had a file size limitation of 8 MB, but there are some compile options that can get that to 33 MB (see the emacs FAQ). As of version 19.29 the limit was bumped up to about 132 MB. As of emacs 20, if you compile it on a 64 bit system, the limit goes up to 549,755,813 GB. Yes, that's almost 550 million Gigabytes. That's a lot of file to edit.