I used to work in a Christian book store and I made Bible covers and sold them
Little Hud has the right idea. But I would suggest that you lay the Bible out open ontop of the fabric and give yourself a little extra in the width. When you get it made you will need a little extra room for when the Bible is closed and most people tend to stuff papers in them so allow a little extra.
You can add pockets in the inside or on the out side. If you want directions for adding a zipper let me know. It is too much info to add here.