The question as to what you would like to plant in the garden may determine your choice of garden size. If flowers are your joy, then a shallow garden would be appropriate while the vegetable garden may require deeper soil for root types of plants. So according to this, a 6 inch tall garden would do well for the flower gardener and a 12 inch tall would do for the vegetable gardener.
Be sure to plant variations of vegetables you enjoy the most. You will immediately be impressed by the difference in the taste of the vegetables you grow yourself over what you experience with store bought food.
Arranging the types of vegetables, or flowers, to best take advantage of the sun should be considered. Taller plants would naturally be toward the back as would trellised vines. Selections similar to tomatoes might grace the center rows while things that grow close to the ground like lettuce and cabbage would be planted in front. This maximizes the sun for each species. Another consideration would be to stagger the planting of each kind of vegetable to different times so that they would bear fruit throughout the season. Don't forget that you may have several planting times when you consider that some plants thrive into the fall and winter.