I plant all of my bell pepper plants and jalapeno plants in three gallon pots. Egg plants likes bigger, but will still grow well and produce in a three gallon pot. Most any herbs will do well in that sized container, and you could really do some groupings of two or three different herbs in each container.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.