Raised garden beds are almost ready made, but you can make one yourself. The size may be just 2 feets or 1 foot because the rats should not spoil your garden bed.
It depends on what you are growing and what kind of drainage system you have. For tubers, you need at least 1 foot, for greens like spinach, lettuce, etc., six inches can be ok.
Another thing to take into consideration is the physical aspect. Some people have raised beds so they themselves don't have to bend down. Those are a couple to several feet high. They then plant the plants accordingly.