Are snowflakes larger when it's very cold or when it's barely below freezing?
Individual snowflakes ... such as stellar dendrites ... are larger when it's very cold; however when it's barely below freezing ... individual flakes aggregate into 'clumps' when they collide as they fall to earth.
Barely below. Snow tends to be dry and flaking when very very cold; so the flakes do not stick together. But when the snow is wet, the flakes will stick together as they fall. And that results in multiple flakes stuck together as one. Grew up in snow country near Lake Erie.