Stairway To Heaven would have definitely been an EP if it had been released as a generel 45...I saw a DJ 45 of it on You Tube once. It's very hard to get 10 minutes onto a disc that should at most hold less than 8 because of the difficulty of cutting it and associated groove problems.
EPs also came on 10" microgroove discs (although they were originally the 10" size LP), while RCA Victor would use 45s and put 4 songs on them (I have a few of these, mostly hits discs reprising artists like Tommy Dorsey). In the late 70s CBS Records introduced both the half-speed mastered 12" LP and reintroduced the 10" record as an EP to showcase their artists, which only served to confuse the changer users and maybe leave some feeling gypped out of 2" (Cosmo Girls, probably) so they were not around for long.
In the 80s EP came to signify a release that had less content than would normally be expected on a full album release, a short compilation of outtakes, or a filler album inbetween regular releases to keep the fans interested and buying.