OMG Witherzilla!!!!, It's giant, it's invulnerable it's unable to move, and it's not aggressive. An ender dragon would be more like the ender dragonfly compared to this guy, a block not even be one pixel, and I would be like the size of a flea. If you want that huge wither boss, do /summon WitherBoss ~ ~ ~

. This works simply because since the Wither is named Dinnerbone, its growth is reversed and instead of turning it into a regular sized wither, starts out as a humungous invulnerable wither at full health. Credit to SimplySarc who discovered the glitch.