Plants – Determining pot size

Q: I have a lantana and a ‘Siskiyou Pink’ gaura that I would like to plant in pots on my deck. Is there a formula for determining the size of the pot necessary based on the average height and/or width of for healthy growth of these or any other plants?

A: My offhand formula is that the size of the container should approximately equal the “volume” that the branches of the plants growing from the container occupy….so it depends on how big the lantana and gaura plants will get.