Walking or stepping on beds is not recommended so it’s a good idea to go for a width of less than 1.5m (5ft). This will allow you to reach it from the sides. For ideal ease of use I would go for 1m wide, or 0.5m if you can only access from one side.
In terms of height, it depends on how you would like to use the bed. These are my recommendations:
Ideally you should always build to at least 0.6m so that the plants can root deeply (which will also reduce their watering needs), but you can build smaller if you prefer.