Figure the O2 output is directly proportional to surface space of the leaves. That is close enough for a first comparison.
Six feet is too deep to reach into and weed and harvest, so figure on making two rows of plants, where you want to be able to walk down the middle.
What you plant has a lot to do with where you are. You can put a lot of herbs in. If you are zone 7 or higher, I'd put in tarragon, rosemary, basil, chives, parsley. Skip the first two if you are colder. Add your favorites, and add something odd, like lemongrass or savory.
Next, choose veggies which are really tastier from your garden than from the store. A good, indeterminant tomato (Sun Gold for cherry tomatoes, maybe Early Girl for larger ones). Beans (I like the true haricot vertes), peas (english for shelling, or snap peas to eat the whole thing), lettuce for an early crop followed by kale after the lettuce is done.
The soil you have is fine. Add compost, especially if you can get it free (some towns collect yard waste and give away the compost) to improve the soil.
