Based on recessional velocity of 8380 km/sec, about 375 million light years away. Given that and apparent size of 0.6 by 0.4 arcmin, about 65 thousand light years in diameter.

A nearly three degree wide emission nebula about 2500 light years from the Earth (and hence the best part of 150 light years across), IC 1396 fluoresces in the intense radiation of the 4th-magnitude variable star near its center, μ Cephei. Bordering the nebula and within it are numerous knots of cooler, denser gas and dust which are outlined by or silhouetted against the hot, glowing gas surrounding them. One of the more dramatic knots is the elongated structure to the west (right) of the nebula's center, called the Elephant Trunk Nebula (closeups of which are shown below, in visible and infrared radiation). μ Cephei, also known as Herschel's Garnet Star, is the prototype of the μ-Cephei type variables -- a red supergiant of irregular brightness, varying from about magnitude 3.5 to 5 over a period of two to two and a half years. Its mass is estimated at 15 times that of the Sun, and it is one of the largest stars known -- the best part of two billion miles across at maximum size -- meaning it would nearly fill the orbit of Saturn. It is a very cool star (only about 3700 Kelvins, or 6200 Fahrenheit), so most of its radiation is in the infrared. Its visible radiation is about 40 thousand times that of the Sun, and taking into account absorption of its light by interstellar dust and the large proportion of infrared radiation it must give off, its total luminosity is nearly ten times greater, or about 350 thousand solar luminosities. The star is near the end of its life, having already begun to fuse helium into carbon in its core, and within a few million years (hardly any time, on an astronomical time scale) will blow itself to bits in a supernova explosion, leaving behind nothing but (most likely) a black hole or (less likely) a neutron star.