A garden "bed" is a formed mound of soil that is raised higher than the surrounding soil. Often times, gardeners build frames around these, but it is not required. When I use the term garden 'bed', I simply mean that formed soil whether or not it is framed out. I often build small framed beds for my clients, however, as it keeps things both orderly and visually tidy. Garden beds come in many shapes and sizes and are made of many different materials, but over the years I found that a 6'x4' bed is the perfect size for a small backyard garden, and here is why. Firstly, when designing a bed, you want it to be 4 feet to 4 1/2 feet wide at most. This width allows you to reach the center of the bed from the rows, on either side. It is an inefficient use of space to build it any wider, as it ends up being dead space.
Secondly, the ideal length for a bed in an urban environment is 6 feet. When growing vegetables, it is best to work through your crops and rotate diffferent plants throughout each bed - this is called a crop rotation. Rotating crops keeps soil fertile and minimizes the proliferation of disease. If the bed is small enough to plant one entire crop rotation (and rotate other crops through the additional beds), it becomes easier to organize and rotate plants over the course of a growing year. A six foot bed helps to accomlish this. The six foot length allows you to plant 8 to 9 large plants in one bed, such as tomatoes, tomatillos and peppers. This is your fruit rotation and a decent amount of space for summer fruiting plants. You can also fit 8 lettuce heads across a four-foot width. Plenty in just one short row.
Four beds but 4'x6' is about 100 square feet of growing space - a lot of room and plenty for a family of four to start.