How far can endermen reach?

Solution 1:

Reportedly at PAX (summary) it was said that they have a two-block reach: you'll want to have a one-block buffer zone between the fence and anything of value.

The reach, based on the pictures I've seen of the Endermen's destruction (and my own experience), appears to be directly in front, 1-2 blocks laterally, and from ground to eye level. Diagram:

Enderman facing to the right -> o ..
                                o ..
                                o ..

o : Enderman body
. : Blocks that are fair game

Evidence of Enderman mischief:

• Two blocks above the ground
• One block above the ground
• Ground level

Coincidentally, this mirrors the one-block buffer zone you need to have for beds in order to sleep undisturbed.

Interestingly, vines might be a better alternative to fences, if you're okay with the appearance: Endermen will take the vine block before taking anything behind it. Since vines automatically regenerate, you'll have a defense that's more compact than a fence with a buffer zone.