tomatoes

as a general rule (which obviously does have exceptions :) ) the size of the roots of a plant is about the same below ground as it is above.

will the plants have room to grow down in there and what type of soil is it?

as for the size of the plants that is going to depend upon what kind of plants they are and the conditions (light, water, weather).

so, um, that could be ok for three small patio style tomato plants. but for one plant of the beefsteaks we usually grow (with sprawl into the surrounding area).

songbird

In diameter,but how about depth? Only has six inches.

as stated, contained area, six inches deep.

and what type of soil is it? As stated, potting mix.