Relative Distance Scales
To set things in perspective, you should recall that the diameter of the
presently observed Universe (twice the
distance to the most distant objects yet observed) is about 10,000 Mpc.
The distance scale associated with the
Great Wall is mind-boggling by our normal standards: it would take light
more than half a billion years
to travel its observed length. However, it takes light more than 30
billion years to cover the diameter of the
presently observed Universe, so even the Great Wall is rather small compared with the size of the known
Universe.
Furthermore, these statements apply to the known Universe. The entire Universe is
presumably much larger (perhaps infinite), but we haven't seen it yet because light from more distant regions
has not had time to reach us since the big bang.