You can still get single rolls, if you buy the cheap stuff. It’s noticeably narrower in diameter. There’s also triple rolls and “jumbo” rolls or whatever they call them, that are noticeably larger than the double rolls. There may have been an industry standard back in the day, but there clearly isn’t now, with TP not only varying in number of sheets, quilting, diameter, but also width and as mentioned above, inner diameter too (when they remove the cardboard tube).