In a day, shadow length is shortest at noon time and longest when the sun is near horizon i.e. sunset and sunrise. The reason for this is that the angle between earth's surface and sun beam is maximum at noon and minimum during sunrise/sunset. read more
Now if we talk about a particular time of a day during different seasons, then the shadow is longest on the winter solstice and shortest on summer solstice [on winter solstice, the day length is shortest]. read more
Shadows change length throughout the day because the angle at which the sun shines on stationary objects changes with the Earth’s rotation. For example, early in the morning, when the sun is near the horizon, it casts long shadows when an object blocks the light. read more