What is the proper format for an autobiography?

I would figure that would be a decision for your editor or publisher to make. Assuming you’re using a editor and going through a publisher.

Yes I am. I just didn’t know if there was a set standard.

Do you mean autobiography (about yourself) or biography (about someone else)?

The publishers I have worked with just want a legible manuscript. They didn’t generally want writers to try to do formatting and layout, which they’ll just end up removing.

But you could go to a book web site that gives PDF previews, pick a few recent popular such books and take a peek at the previews to see how they formatted them. Anything close is probably fine.

Also, publishers often have submission guidelines which may tend to tell you what file format to send and other suggestions. Following as many of those as closely as possible will remove at least those reasons for them to ignore your submission without reading it.

You’ll want to write it long before you come to a discussion of font size and style. Maybe take a memoir-writing class or just read some (many) autobiographies and think about their structure, approach, and character—not always strictly linear or chronological, and not always uniformly anchored in the framework of time and place. Some are more personal and episodic, and some are a mirror to the culture and history of their time.

One thing publishers want is an answer to the question “What’s so special about this guy?” or “What gives this woman distinction?”—in other words, not just what will interest a reader but how they are going to market it: overcomer of great obstacles, inventor or discoverer of something important, influencer of culture, power behind a throne, survivor of a major disaster, that sort of thing.

In other words, what is the audience, scope, and purpose of the book, and where’s the manuscript? All that comes way ahead of book design, which in itself is a different matter if the book is not being printed in hard copy.