For tomatoes their roots go deep and they are THIRSTY and HUNGRY plants, so the bigger the container the better! I'd say minimum 5-gallon bucket, regardless of the variety. You really can't get them in too big of a container unless you're planting one plant in an old bathtub or something silly like that

Just make sure that the container has good drainage (poke several holes in a 5-gallon plastic tub, etc).
When you plant the tomatoes, look at the plant first. Is it short, compact and bushy or is it long and kinda skinny (also known as leggy). If it's nice and compact it means you have a great healthy plant...plant it in the ground as is. If it's leggy, what you do is you leave the top set of leaves on the plant. Any other leaves along the stem you should pinch or cut off right next to the stem. Now you have a really long stem and just top leaves. When you plant it in the ground or the container, plant it so the ENTIRE stem is buried except for 2 or three inches sticking out of the ground. Tomatoes are fantastic because unlike other plants that will rot when the stems are buried, tomatoes will produce roots all along the buried stem. You will want to make sure to do this so the tomato plant grows strong, otherwise you'll have a skinny leggy plant that will snap at the base during the first strong breeze.
The only other thing to make sure is that they are watered enough (once a day, at least...the soil should never go completely dry and it's really impossible to water tomatoes too much as long as they have drainage) and fed enough. Earthworm castings are my favorite fertilizer but you can buy tomato food or anything. Just follow package directions. They WILL need some kind of fertilizer or plant food because they'll use up whatever's available in the potting soil pretty quickly, even if you have that fancy "feeds for up to 9 months!" potting soil
Also, contrary to popular belief, you do NOT have to cage your tomato plants. Just make sure the fruits aren't sitting against wet soil (so they don't rot) and you'll be golden.
Hope your plants are still alive at this point!

You can always replant them deeper, too. Just dig up the current plant, make a deeper hole, and voila!
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