How far was it along the Silk Road from Chang'an to Dunhuang? To Marakanda? To Antioch? To Rome?

The Silk Road is very hard to be measured, and this is due to its structure, which has both land routes and maritime routes, as well as having multiple routes that branch out in several regions. So to make an estimate about the distance between Chang'an and Rome is almost impossible because there's both land route and a maritime route from Levant and on.

It is estimated that it was approximately 8,700 km long, but as I said, if we take the whole network of the Silk Road routes than the combined distances may rise to approximately 35,000 km. It was a network of routes that were connecting East Asia, Central Asia, Southeast Asia, South Asia, the Middle East, the Horn of Africa, and Europe.