Appropriate pot size is determined in part by the current size of the root/soil mass, but primarily by your choice of grow medium. The more water-retentive your grow medium is, the more critical pot size becomes. This is because it's the water held between soil particles in a saturated layer of grow medium at the bottom of the pot that limits root function and often sets the stage for infection by fungal pathogens that cause root rot. When using media that hold little to no excess water between soil particles, there is no upper limit to the size of pot you choose. With the right grow medium, you can plant a single seed in a 55 gallon drum filled with soil w/o concern that you've over-potted the plant.
When you DO have a pot you recognize as being too large for the plant material it holds, based on the fact the medium is too water-retentive for the size of the pot, judicious use of ballast in the bottom of the pot can eliminate (displace) as much as 95% of the grow medium capable of holding excess water, which can be a real game changer for
anyone who finds their grow medium holds enough water to cause concern about root function/health.
The line drawings below show ways to reduce water retention in your plantings. The pots are full of grow medium and the shaded area represents the layer of saturated media (perched water) I mentioned. B, shows how much additional water you can remove by simply tilting your manageable size pots to a 45* angle after watering.
C, shows how a pot's shape can affect the amount of excess water it's capable of holding, while D shows one way of using an overturned pot as ballast to limit water retention, and E shows how effective using a drainage wick can be. There are also other ways to use simple science to 'trick' water that would otherwise remain in the medium into leaving the pot.
In terms of growth and health, the larger the pot you can use w/o excess water retention becoming an issue, the better the opportunity your plant has to realize as much of its genetic potential as possible. Roots are the heart of the plant and a healthy plant isn't possible unless we can keep its root system healthy.
Al
