They were always 3.5-inch
Correct Answer: 8-inch

A little background…

Invented by Alan Shugart at IBM in 1967, the original floppy disk design measured 8 inches (200mm) in diameter, stored 80KB of data and became available for purchase in 1971 as a part of IBM's products before being sold separately in 1972 by companies including Memorex. In fact, Shugart himself left IBM and transitioned to Memorex in 1972, helping the company deliver the first commercially available read-write floppy disk drive (the Memorex 650).

Answering demand for a smaller format diskette that was more manageable and affordable for desktop word processing machines of the day, Shugart's team designed a 5.25-inch 'minifloppy' disk in 1976 that stored 87.5KB and within two years, more than 10 manufacturers were making 5.25-inch floppy drives.

The shrink to 3.5-inch disks came in 1982 after a consortium of 21 companies agreed to specifications, which included a formatted disk capacity of 280KB (soon increased to 720KB and eventually reaching 150/200MB by the late 90s).