How big is a crab nebula?

This is a horse head nebula if that helps.

http://www.nasa.gov/images/content/52238main_MM_image_feature_89_jw4.jpg

Even though the Crab Nebula is the focus of much attention among astronomers, its distance remains an open question due to uncertainties in every method used to estimate its distance. In 2008, the consensus is that its distance from Earth is 2.0 ± 0.5 kpc (6.5 ± 1.6 kly). The Crab Nebula is currently expanding outwards at about 1,500 km/s.[14] Images taken several years apart reveal the slow expansion of the nebula,[15] and by comparing this angular expansion with its spectroscopically determined expansion velocity, the nebula's distance can be estimated. In 1973, an analysis of many different methods used to compute the distance to the nebula reached a conclusion of about 6,300 ly.[3] Along its longest visible dimension, it measures about 13 ± 3 ly across.[c]

Tracing back its expansion consistently yields a date for the creation of the nebula several decades after 1054, implying that its outward velocity has accelerated since the supernova explosion.[16] This acceleration is believed to be caused by energy from the pulsar that feeds into the nebula's magnetic field, which expands and forces the nebula's filaments outwards.[17]