Mine are 2ft x 2ft and I use them for growing raspberries and trailing blackberries. Both seem to grow fine in the raised beds. I usually add a thick layer of straw or pine needles on top of the bed to regulate the moisture.

I use 4x4 for blueberries and we had 13 days below zero. All three fruited, and produced a small crop. They are all rather young plants still. I also mulch heavy. No doubt the roots were frozen, but they did fine.
Here is a recent photo. The red plants, if you can see them are the blueberries. In front of them is a 3x4 bed. Three 4x4 and three 4x3 in front. a 25 inch x 24 ft bed to the left of them for raspberries. Three 1x4 beds for beneficial flowers. Other beds there too a 12x8 (for strawberries)and against the shed a 3x12 (rotate beans and pepper or tomato and garlic plants).
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This post was edited by Drew51 on Sat, Nov 22, 14 at 23:38

Raised beds are large containers. The best benefit is drainage. Also you use potting mix to replace the native soil.

To best use the raised bed materials, build it at least 4' wide.

Most of the folks build the raised beds on existing soil or turf. Then they put on about 1' deep potting soil. There is a big difference between the potting mix and the existing soil. So raised beds are excellent for annual vegetable since the roots do not go deep.

This is also good for certain perennial fruits, such as blueberries since most of the roots stay at the first 6" of the soil. But certain fruits, such as grapes, set very deep roots. So the potting mix and soil separation is not good for grape vines.

Grapes like crappy soil anyway, best in ground. You can do things too like dig it deeper. Also I would not use potting soil, I would use garden soil. You can mix the native and garden at the bottom. I do like recycling potting soil into raised beds. I add compost every year, as mine are mostly organic matter and it is eaten up every year, well almost. I have lost as much as half the height from decomposition of organic manner. If using them for shrubs, mound the shrub as high as possible. For blueberries instead of compost I add pine bark every year. If building for blueberries I would use 1/3 pine bark, 1/3 peat, and 1/3 garden soil. Mound the plant in the middle as high as possible,

You can certainly use regular garden soil, but you do not get most of the benefits of raised bed.

Raised bed is primarily good for the folks who do not have good native soil. The native soil can be too heavy, or too light, or have certain diseases. And some folks do not want to dig, so we have the raised lasagna or no-dig bed. Also, some folks have small garden area (square foot garden) and want the most production out of the garden bed.

If done correctly, a lasagna bed can be super production, with 50% or more with rich compost, or any other improved soil. So the soil is as light as the potting soil, but much richer. So the yield can be very high.

But again, do not plant deeper than the raised bed soil line, or you'll get something like this:

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This is true for tree, shrub and others.

Raised bed can be very good, but it is not practical with a large garden area. If native regular soil is used, then it is much easier to mount the regular soil and create good drainage.

My soil is good so found the plants have no trouble going right through to it. My trees are not in raised beds, but they look nice in beds. I like the beds as the soil warms up faster, so melons and corn and other things that like warm soil can be started earlier. Also in wet areas you can increase drainage with raised beds. I like it too because it defines a clear area for my brambles to grow. Any that stray out are removed. I will always use raised beds, the advantage is too great.
I have another reason that is more specific to me. My dog has a hard time telling the difference between garden and grass. But it was easy to train him to stay out of the raised beds. I have to grow all vegetables in raised beds else my dog would stomp them into the ground. I grow blackberries in the ground and had to put a fence around them to keep him out. I could not train him to stay out, he didn't get it? The green posts with white tips hold a chickenwire fence, so he cannot run though the blackberries, which are currently buried under leaves for the winter. Burlap will go over them too. Putting it down Tuesday.
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Jesse will go right up to the raised beds and sniff in them, but is trained not to walk in them. (Chandler blueberry in full fall color)
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This post was edited by Drew51 on Sun, Nov 23, 14 at 18:22