It’s a more difficult question than you might thing. This is something I wrote for NightFlight.com.
It’s an easy question for the casual vinyl junkie. “Records are either seven inches in diameter if they’re a single or twelve inches if they’re an album or an EP.”
Really, now? Are you sure? And while we’re at it, do records even have to be round?
When we made the transition from Thomas Edison’s cylinder to Emile Berliner’s rotating disc about 120 years ago, the standard diameter for these newfangled things was generally 7 inches. No reason; it just seemed like a nice size. But in 1901, some wiseass decided that 10 inches was a better side. A wider record meant that the grooves could be spaced out more widely; wider-spaced grooves meant that the grooves could be deeper; and deeper grooves that more information could be stored within then, leading to better sound.
But if 10 inches was good, 12 inches had to be better, right? That size started making its appearance in about 1903 and for some reason spun at 60 RPM instead of the generally agreed-upon 78-ish RPM (it depended on the phonograph manufacturer, the record label and if you were in North American or Britain–let’s not get into the voltage issue here. In the end, though, 78 became the standard worldwide).
Other labels tried different sizes: 8 1/2, 9, 11 3/4, 14, 16 and even 20 inches, making it all very silly for a while. Fortunately by the time we got to WWI, clearer heads had prevailed and the world settled on 78 RPM 10-inch discs, which became the standard for the next forty years. It wasn’t until the late 40s that the 78 was pushed aside by the 33 1/3 12-inch album (introduced by Columbia Records in 1948) and 7-inch 45 RPM single (a creation of RCA in 1949).