linda mar wrote ...

1. how large diameter would be sufficient to give enough root space for
most plants?

Depends on whether or not you knock the bottom out of the pot and/or how
often you're willing to move plants around. If a plant gets too big for one
of my pots, I either replace it with a smaller plant or take it out and
prune the roots.

2. how shallow can these containers be?

Not sure I understand. Just deep enough to hold your plants and planting
medium?

3. do I need to punch any holes in the bottom of the containers?

Usually I do, but sometimes I don't. Obviously water circulation is better
if you do but, on the other hand, their easier to move around if you don't.
Experiment.

4. wouldn't the roots rot?

Nope.

5. can you do this with larger plants like amazon sword plants, etc?

Yes and, actually, those are exactly the sort of plants I do use pots for.
Amazons are heavy root feeders - by confining them in pots I can provide
fertilizer spikes, without having them leach away into the rest of the tank,
and special planting medium, such as peat moss.

kush