In the 1890s, the recording formats of the earliest (toy) discs were mainly 12.5 cm (nominally five inches) in diameter; by the mid-1890s, the discs were usually 7 in (nominally 17.5 cm) in diameter. By 1910 the 10-inch (25.4 cm) record was by far the most popular standard, holding about three minutes of music or other entertainment on a side. From 1903 onwards, 12-inch records (30.5 cm) were also sold commercially, mostly of classical music or operatic selections, with four to five minutes of music per side. Victor, Brunswick and Columbia also issued 12-inch popular medleys, usually spotlighting a Broadway show score. However, other sizes did appear. Eight-inch discs with a 2-inch-diameter (51 mm) label became popular for about a decade in Britain, but they cannot be played in full on most modern record players because the tone arm cannot play far enough in toward the center without modification of the equipment.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What was the most popular sized disc by 1910?
10-inch