Wither skeletons spawn in Nether fortresses at a light level of 7 or less. They spawn in place of a regular skeleton 80% of the time.
Wither skeletons are just over 2.5 blocks high and require 3 vertical blocks of space to spawn.
On Halloween, wither skeletons have a 22.5% chance of spawning wearing a pumpkin and a 2.5% chance of spawning wearing a jack o'lantern.