The expansion jontis are formed either by you putting thin wooden slats in that leave slots, or, while you are smoothing the surface, you actually shape the slots with your trowel.Usually the individual section can be about 4 x 6 , and larger if you use heavy duty steel grids to strengthen the concrete.With your measurements I would do one down the center, and the others at 6 to 8 intervals.Since this also depends on the quality of the concrete you'll use, you may want to double check that with your concrete supplier.Depending on the size of the vehicles you'll be running across that, I would think that you want at least a 6" thickness, and more if the underground isn't very hard and stable. Use one to two inch gravel as a base.I did that on a driveway 38 years ago and it is still perfect.