"giant snowflakes" means very big snowflakes falling from the sky during winter. It suggests that the snowflakes are larger than usual. Full definition
The DVD «Desert Blizzard» (1996 - 97) records giant snowflakes from a skywriting plane falling in the bright blue of cloudless desert sky. (brooklynrail.org)
Cross-country skiing There's something magical about gliding through a silent forest under your own power while giant snowflakes twirl down from the sky. (skifernie.com)
In 2006, for instance, at the Renaissance Society in Chicago, he installed 100 live pine trees and giant snowflakes sculpted from marble, crystal, and papier - mâché. (artnews.com)