TRACE gives us the most detailed view yet of loops in the solar atmosphere. Its powerful telescope allows us to see loops that are 'only' 1,000 km across. That's like being able to look through a telescope in Glasgow and see Nelson's column in London.
Here are some amazing pictures of solar loops that were taken by TRACE. The loops are up to 100,000 km long.
Here's a group of loops at the edge of the Sun (known as the solar limb). They fan out because the magnetic field fills all the space it can. Loops of many different sizes and brightnesses are present in the same group. The width of the loops you can see is about 1500 km. Thinner ones probably exist, but we'd need a better telescope to see them.
Here are some loops in a different active region. Again, the colours are false in this picture, with the white showing the hottest coronal plasma and pink the coolest. The green patches are sunspots. Notice how some of the loops appear to have one foot in the sunspot.