20 gallon is a good size. Some varieties - dwarfs and determinates can get by with 10 gallons, some dwarfs can even do well in 7 gallons I find. But I find that the big indeterminates want 15-20 gallons.

Dave

Yea, 20 gal would be great. Just too much soil for me to deal with though, so 15 gal max for me and some smaller.

As crazy as this sounds, I don't know if there is that much of a difference in volume - to some degree. For example, I've repotted a 10G into a 30G root pouch. When I added bottom and side fill to the 30G, I feel like the 10G plant plug was already 80% of the volume.

The problem with tomatoes is that they don't really efficiently utilize the pot volume. Peppers are much better at this for example. Tomatoes tend to just go around the edges and the bottom, while about 1/3 of the volume just above the bottom has no roots even in rootbound conditions (verified this myself).

Using airpruning pots should help a lot in theory (never had the chance to try yet).

There is a sweetspot of size vs production, but that depends on the variety used, watering regime (you can get away with much smaller with drip irrigation) and growing techniques (like pruning). For a one stem tomato 7 gal is just about right, for unpruned, hard to say, my guess would be around 15.

I think finding the right varieties for the container is more important than the container size, but I liked having three small tomato plants in an ~18 gallon moving tote. ~5 gallons worked fine for a dwarf-ish cherry I had. 10 gallons didn't seem like enough for another plant I tried. You can grow larger plants in containers, but I think smaller varieties shine in them better, more often. Some tomatoes like more space.

The reason could have more to do with leaves than roots. Plants with more leaves tend to use more water (although there is some variance between varieties here). How fast the containers dry out may impact the fruit.