The tooltip for Rend says:
"Causes all nearby enemies to bleed for 210% weapon damage over 3 seconds."
How far is "nearby"?
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Rend without any runes only will affect enemies within the sphere of your hitbox, which is about 10 yards.
The first Rune that comes available for Rend, entitled "Ravage" increases the range of Rend to all targets in a 17 yard radius making it significantly more effective against large groups of mobs surrounding the barb.
You can get an idea for exactly how far this actually is in game by checking out this great post.