I grow them every year from seeds, too, but only realized this past year that they formed tubers! Not sure how big the tubers themselves get, but they don't seem to like growing in pots, unless the pots are pretty big. My mom grew 4 O'clocks year round in SoCal and they got to be around 4 feet tall there! Mine have done best in planters we built that were about 18" deep and roughly 20" x 40."
I always soak the seeds, but haven't needed to nick them.
And I agree with Allison, they do better when you keep potting them up.
