During the day, the sun is highest in the sky at noon, so shadows are shortest. As the sun rises and sets, shadows get longer. This is because the sun is at a lower angle, so the rays of light are more spread out and create a longer shadow.
The winter solstice occurs in the Northern Hemisphere in late December, and the summer solstice occurs in late June. On the solstice dates, the sun is farthest north and farthest south, respectively. This means that the sun is lowest in the sky during the winter solstice, so shadows are longest. The sun is highest in the sky during the summer solstice, so shadows are shortest.
So, the length of a shadow depends on the time of day and the time of year. Shadows are not necessarily longer in the summer than in the winter.