Change default indent size in mail.app
This is normal.
(It cannot be changed)
The standard tab size for an 80 column page (typewriter or console) is 8 spaces.
You also must remember, that email is for sending correspondence (letters)  and indentation here is &quot;old fashioned.&quot;  In other words, you shouldn't be indenting in your letter writing.

What Is an Indented Letter Style?
How to Write a Letter

In the second link, you do indent, it should be 1&quot; and that is approximately 8 characters.