got scale with plane and empty on array

I used Blender version 2.77a
I add one plane , change it.
I also add one empty with arrows.
The empty and plane is on (0,0,0).
I add one arrow modifier to plane with object offset the plane is sized wrong.
The blend file:

You changed the plane’s size in Object mode. As a result it now has unapplied, non-uniform scale:

Select the plane in Object mode and hit Ctrl-A > Scale to apply the scale to it. The scale values should now read 1.000 / 1.000 / 1.000.

Rule of thumb:
As long as you are in the modeling / sculpting / texturing phase, always make sure the scale is 1 / 1 / 1 by either changing an object’s size in Edit mode or by applying the scale with Ctrl-A.

Thank you. Yes. I forgot. I have not used too often this option , although it is universal step.