Testing for players within a specified rectangular area
The delta parameters (dx, dy, dz) state the number of blocks from the origin (x, y, z) to the opposite corner of the rectangular area, rather than a second set of coordinates.
For example, the following looks for players with the first corner at (100, 64, 100) to the opposite corner at (105, 70, 105):
/testfor @a[100,64,100,dx=5,dy=6,dz=5]