A CD is 120 mm in diameter. Why were they designed to be that size? Why not 130 mm?
Note that CD-ROM drives fit into a standard (so-called) 5" bay. In terms of 1980s tech, a wider CD would probably have meant that the drive would have been too wide.
I also presume that the limitations of data density of the time meant that a *smaller[/] size could not be made and still hold a standard classical album. (Which is what dictated the 12" size of LPs.)
There’s some interesting stuff about the development of the CD specifications here snopes: roll over, beethoven
The implication made was that the 12 cm diameter was near the maximum size that could be ‘put into a suit pocket’, which was Sony president Norio Ohga’s minimum standard for portability.
ftg: was getting CD drives to fit in computer bays really that important a consideration in the late 70s-earliy 80s?? I always thought that the CD standard was evolved purely as a home stereo thing, and then got retrofitted to computers. (Maybe that’s just because we didn’t have a computer with a CD-rom drive until 1996. 
But then, we didn’t have a home stereo CD player until nearly the same time.)
This site claims that discs were originally set at 115mm back in 1978, but in 1979 increased to 120mm to fit 74 minutes of 16 bit stereo sound at 44.kHz sample rate. It also lists 1985 as the year that the CD-ROM entered the computer market.
Not many operas fit on a 74-minute CD. Most uncut operas take up two or three discs.