I don't think they're too big at all, as long as you have enough dirt to fill them. :-)
Something that size allows you to plant multiple plants in the container. I like growing green beans in containers, the bush type, not pole beans. Growing in containers makes them easier to pick and the plants don't end up laying on the ground and getting the beans dirty before you pick them. I also grow tomato plants in big pots, as well as pepper plants. I'm sure there's a lot of other veggie plants that you can grow in containers also.