There are lots of things to consider before building a raised bed in the garden. Think about the height that you want it to be and make sure that it is easy to access. You may want to consider making a seat along one or more sides of the bed. In addition, you need to think about the exact location in your garden.
In order to be able to easily reach from both sides of your bed, it needs to be no more than a metre wide. The length of the bed will be determined by how much space you have available. If you can only reach your bed from one side, then the maximum width will be about half a metre.
The height of your bed can be determined by your gardening preferences. Do you like to stand, sit or kneel when gardening? If you are constructing your bed for use by someone in a wheelchair then your bed needs to be constructed to a height of about 62cm (approximately 2′).