Hi Nadine.
I think 20' is a good estimate, tho it really depends on conditions it is in. If they are optimal, with regular water and plenty of sun, it might get bigger. One important thing to note: these trees virtually ALL develop anthracnose over time -- a disease that can ultimately kill the tree, I believe. Also, they cannot tolerate pollution, salt or drought. Other than that, they are great, gorgeous, 3-season trees. If you are worried about anthracnose, you could try Cornus kousa instead -- another dogwood -- with a later bloom time and later fall color, and pretty much disease-free. I've killed 3 magnolias at my house, so I can't give advice about them. You might also look into Stewartia koreana, which is another gorgeous small 3-season tree.
Good luck.
ML

HI guys, checking the notes and will simply say I like them. Just a note, your size ratio is realistic at 18-24' mostly on the horizontal plain. Remember some good news. Pinks seldom exist on there own, they are grafted on to white root stock. That is where your size ratio changes. New processes are working in growing stem cuttings, but seeds seldom remain genetic. Another good point to remember , trim.... just wait for the blooms to happen and cut back the unwanted length or problems. Do this before the regrowth sets in , that's next years pretties. Good luck.