I am interested in knowing how big peonies get? How tall and how wide? Also do you get more impact in mass plantings or do they stand alone well?
Mine get to about 3 feet high and wide. They look good either as a mass planting or stand alone shrub. Al
You can buy all sizes from 8" to 36+" Look at some of the Alpine-rock garden sites for suggestions. Most suppliers give the average height of the mature plant. Depending on your soil the height could be more or less.
Since I have a preference for many different plants I look for something other than just the bloom. When you can look at the foliage. Some growers will show a growing plant and others just show the bloom. After the bloom season I want a plant that will make a statement in the garden not just something to cut down or cover up. I like the big bold leaves of a macrophylla sp? parentage all the way through to the finely cut ones with teni parentage. Go to your local garden or look at other peoples gardens. Ask questions. Gardeners like to talk about their likes and dislikes. Who knows you may find a source for a particular cultivar right in the next block.
A "typical" herbaceous peony will be 2'-3' tall and wide, and will make quite a statement on its own, at least for the few weeks that it is in flower. If you have a lot of space you can do a mass planting. I'm not sure you would choose most herbaceous peonies for the foliage. It can be attractive enough, particularly before flower, but tends to become increasingly floppy and ratty during the summer. Tree peonies tend to have more interesting foliage but it varies a great deal, from some that look almost like a herbaceous peony to some that have beautifully layered and cut foliage. Perhaps you should interplant with something else to carry the bed through the summer, since you can cut the peony foliage down early.