Several of mine are more than four feet diameter. How fast they reach this size will depend on the variety and how well they are grown. Years ago when looking to buy a new house, I complained to the realtor about the shoddy construction. His reply was that "the average home buyer only keeps a house for five years before selling and moving on". If that is true(it is not for me)then you need not worry about spacing your peonies. Peonies more than fifty years old without being moved is not unusual. Al

I have over 200 peonies in my yard. All but 3 are 5 years old and younger. Some are 4 feet in diameter (leaf tip to leaf tip)and some are smaller, depending on the cultivar and growing conditions as Al said. I have 3 peonies that are 65 years old and very much in need of a move as they are now shaded. They are probably 4 feet across easily. I think I spaced my newer ones at least 3 feet from each other.

I suggest while waiting for the peonies to fill in you can put annuals in the open spaces.