Brian Monetti, I spent several years working in home improvement and construction
In building a home fire pit, make sure you take all safety precautions! This includes keeping it a safe distance from your house, making it out of non-flammable materials, and keeping all flammable materials such as plants, grass, and woodpiles a safe distance away (10+ feet.)
If you are making it out of rumble stone, 12 pieces per level tend to make a nice sized pit and the pieces will fit well together. You will probably want it to be about 4 levels high. This depends on how large the pieces are though, because they come in different size. Be sure to stagger each level so the joints don't align vertically from one level to the next in order to keep it from falling down.
I would recommend going to a building supply store such as Home Depot. If you ask in the masonry department, they should have no problem letting you lay out a few pieces to see if the sizing makes sense for your application. Good luck!