If you use the target nearest enemy key often and find it not selecting targets because they are too far away, go to the World of Warcraft/WTF/ directory and edit the config file.
Append the following line:
SET TargetNearestDistance "40.000000"
You can also enter other ranges depending on how far away you want to be able to target mobs.
Append the following line:
SET TargetNearestDistance "40.000000"
You can also enter other ranges depending on how far away you want to be able to target mobs.