I just grew this in a container and it did quite well and grew very quickly. Make sure the container is at taller than 12" because it can grow that long.

Thank you for the advice, but I'm a little confused.

You said the container should be at least 12" tall. However I'm not sure what this means. Are you referring to the height and length of the container, or the depth (how deep....as in it has to be 12" deep). Or are you referring to all three of what I just listed having a minimum of 12" (length, height, and depth).

I was just planning on growing daikon in circular potters. What should the diameter of the rim need to be (12", 14", etc.)

The radish can grow to 12" long so you need a pot at least that deep so the roots will have room to grow. There are shorter varieties too.

The diameter of the pot is not so important as having a deep pot.

I think you will be fine spacing your plants 4-6 inches apart.

Use fine textured soil with no rocks or solid chunks which can make the roots not grow straight.

kvenkat is right. The pots should be at least 12" deep. Daikon is about 3-4" diameter so 4-6" apart should be fine. They grow like a long, fat carrot.