Definitely wait for measurements. There is no 'standard' size.
After making a few, if it is a soft cover bible, I would suggest slipping a stiffener inside the pocket with the bible flap after the cover is completely finished. (A piece of cardboard, a piece of the stuff used to make purse sides more stable, etc). If it is a hard cover, you should be fine without it.
But no, they aren't hard especially once you have the dimensions of the bible.