The asthenosphere has no exact size, but the United States Geological Survey estimates it is approximately 111.85 miles (180 kilometers) thick. It is highly ductile meaning that it can change shape and therefore thickness depending on conditions.
The asthenosphere is located just below the lithosphere, which is made up of the Earth's crust and the upper part of the mantle. The asthenosphere makes up the remainder of the mantle.
The reason that the asthenosphere is so ductile is due to its proximity to the intense heat generated by the Earth's core. The geothermic heat created by the core keeps the rock in the asthenosphere in a semi-liquid state at all times.