A: Guinness World Records states that the largest snowflake ever measured was 15 inches wide and 8 inches thick, observed in 1887 at Fort Keogh, Montana. Ranch owner Matt Coleman took the measurement and he later described the snowflake as being "larger than milk pans" in the journal Monthly Weather Review journal.
When a Google search was done for this answer, several sites declared that the largest snowflake ever recorded was 8 inches wide, but no attributions were given for this record.
However, the National Snow and Ice Data Center reports that since routine, official measurements of snowflake dimensions are not taken, the exact answer can't be known.
Snowflakes are agglomerates of many snow crystals, most of which measure less than a half-inch across. Under certain conditions, usually requiring near-freezing temperatures, light winds, and unstable, convective atmospheric conditions, much larger and irregular flakes close to two inches across in the longest dimension can form.
(Answered by Doyle Rice, U.S.A. Today