I built a box that is 6 foot long, 2 feet wide and 2 feet deep last year for the balcony in my apartment. It's on legs so it's about flush with the wall so the sun hits it well. I lined it with plastic, and drilled holes in the bottom for drainage, then mounted a piece of wavy plastic on an angle underneath to catch any water and deposit it into a bucket on one end.
I plan to grow some small vegetables and herbs, using the square foot gardening technique. Some plants use a whole square foot to themselves, while others you can plant up to 16 plants per foot. You just have to be careful not to plant some plants beside each other as their roots don't get along well, and to not plant the same things in the same spots time after time, because the soil will be spent.
This gives me 12 square feet, probably enough for my family for a small garden.. My balcony is 19 feet long, so if I want I can build 2 more of these.
But one at a time, let's see if I can actually remember to water everything first for a full year. lol.
I can't grow peppers really, not enough sun during the day.. I only have south western sun access :/