Raspberries are likely to create runners and pop up outside the box. I did a slightly raised bed of compost, peat moss and clay and they are growing like crazy this first year. As for drawf fruit trees, afaik, the drawf part is the height potential of the root stock, not the root system. I think your proposed bed would be too small, but you are correct it would help with drainage.

I would be interested in what others think, and maybe could comment on having the roots of a tree higher up more exposed to frost...

so how large of a raised bed would i need for a dwarf fruit tree?

Wow, this is purely a guess, you might want to post this in the fruit forum here.

From this they say to use a 18" hole.

The roots of trees will grow many times the height of the tree, so it won't be contained in your sqft anymore.

Is it possible that you can plant this tree on a slope so that there isn't standing water? That might help. I would think that if you made a large hole, used a 50% original soil/50% peat, you would have a good draining hole which is not so great that the tree's roots will circle around the hole.

Let us know what you decide on.