Set the Margins
Each Text
Box uses margins to set the distance of the text from the borders.
Try
This: Format The Margin
Select the
Text Box border.
Go to
Home ->Drawing.
Select:
Options, the small arrow in the bottom right corner of the
Drawing group.

What Do
You See? By default, the Internal margins
are 0.1" on the left and right and 0.05" on the top and bottom.
The
options also include Autofit. The text size will Shrink
if you type a ton of words. The text will also Wrap inside
the shape.