At the very least you want a 5 gallon pot. A lot of people use home depot or lowes buckets because they are cheaper than actual garden pots. All you have to do is drill a bunch of holes in the bottom of the bucket and before putting in your soil of choice add a few inches of gravel, river rock, lava rock etc. To aid in drainage. On the top few inches add some form of mulch to help with water retention. Your plants will be happy happy happy!
Enjoy!