Before building your raised bed a few bits of information are needed. Have you picked the location for your home garden? Once you have a potential location for your garden you need to watch it for a day and make sure it gets a lot of sun. Sunlight is food for plants, the more sun your garden plot gets the better your dinner will be in a few months.
The next question to ask is why a raised garden instead of just plowing into the earth? If your soil lacks fertility or has poor drainage, it is often easier and cheaper to build a raised bed instead of trying to correct a problem with your home lot.
Clay soil drains too slow and sand drains too fast, most people have less than perfect conditions so I am assuming you do as well.
I like to have my filler garden soil purchased before I start building a raised bed. If I don't I tend to get distracted and have an empty garden bed for way too long. It takes more soil than you might expect for you to fill a bed, especially if you are building a large vegetable garden. Soil for a Raised Bed will explain how to get the soil to fill your raised garden bed.
The next thing you need to check is that you can get to all parts of the garden bed to weed. Make sure you can reach the middle from all sides, or the back of the garden from the front and sides. If you are building against a fence or a house have a narrow bed first and make larger beds that are not against anything.
The size of your garden beds will be determined by the types of crops you are going to plant and the number of people you are trying to feed. Make a list of what vegetables your family will eat, don't bother growing broccoli if no one will eat it. It will just end up in the compost pile congratulations you've just made expensive compost.
Crops that are large will of course take more space than smaller ones. Carrots do not take much space but potatoes do. Cramming a bunch of larger crops into a smaller space will not create a good garden you'll just end up with a bunch of stunted funny looking vegetables, if you get any at all.
Proper planning will save you a lot of aggravation in your raised garden vegetable beds so plan first and build second. You can't go wrong with that.
Growing vegetables is a challenge but it is worth it, nothing tastes better than food that you grow yourself. It is also cheaper once you have put out the money for building that is an expense you will not have to spend again, unless you decide to expand.
The very beginning of raised garden vegetable beds has to come before the building or it will probably have to be redone, which you don't want to do.