I wrote this several years ago. It addresses the factors most important in choosing a pot for your plants. I hope you find it useful.

Choosing an Appropriate Size Container

How large a container ‘can’ or ‘should’ be, depends on the relationship between the mass of the plant material you are working with and your choice of grow media. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of a basic understanding about the relationship we will look at, which logically determines appropriate container size.

It's often repeated that you should only move up one container size when "potting-up". The reasoning is, that when potting up to a container more than one size larger, the medium will remain wet too long and potentially cause root rot issues, or at a minimum, limited root function. In reality, it is the size/mass of the plant material you are working with, and the physical properties of the medium you choose that determines both the upper & lower limits of appropriate container size - not a formulaic upward progression of container sizes. In many cases, after root pruning a plant, it may even be appropriate to step down a container size or two, but as you will see, that also depends on the physical properties of the medium you choose. It's not uncommon for me, after a repot/root-pruning to pot in containers as small as 1/5 the size as that which the plant had been growing in prior to the work.

Plants grown in ‘slow’ (slow-draining/water-retentive) media need to be grown in containers with smaller volumes so the plant can use water quickly, allowing air to return to the medium before root issues more serious than impaired root function/metabolism become a limiting factor. We know that the anaerobic (airless) conditions that accompany soggy media quickly kill fine roots and impair root function/metabolism. We also know smaller medium volumes and the root constriction that accompany them cause plants to both extend branches and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have your sights set on.

Conversely, rampant growth can be had by growing in very large containers and in very fast (draining) media where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers in a medium too slow and water-retentive. This is a key point.

We know there is an inverse relationship between media particle size and the height of the perched water table (PWT) in containers. A PWT is water that fills spaces between soil particles and defies gravity by refusing to leave the pot. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, media will no longer hold perched water. If there is no perched water, the medium is ALWAYS well aerated, even when it is at container capacity (fully saturated).

So, if you aim for a medium (like the gritty mix) composed primarily of particles larger than 1/16", there is no upper limit to container size, other than what you can practically manage. The lower size limit will be determined by the medium volume's ability to allow room for roots to ’run’ and to furnish water enough to sustain the plant between irrigations. Bearing heavily on this ability is the ratio of fine roots to coarse roots. It takes a minimum amount of fine rootage to support the canopy under high water demand. If the container is full of large roots, there may not be room for a sufficient volume of the fine roots that do all the water/nutrient delivery work, and the coarse roots, too. You can grow a very large plant in a very small container if the roots have been well managed and the lion's share of the rootage is fine. You can also grow very small plants, even seedlings, in seemingly inordinately large containers if the medium is fast (free-draining and well-aerated) enough that it holds no, or very little perched water.

I have just offered clear illustration why the oft repeated advice to ‘resist potting up more than one pot size at a time’, only applies when using heavy, water-retentive media. Well-aerated media are not bound by the same restrictions. As the ht and volume of the perched water table are reduced, the potential for negative effects associated with over-potting are diminished in a direct relationship with the reduction - up to the point at which the soil holds no (or an insignificant amount) of perched water and over-potting pretty much becomes a non-issue.

Al