I had thougth about maybe putting a box there that would let you drag and item in there, and it would tell you how much the item was worth, and what the item was made of, so you could get a general idea of what is needed to craft that same type of item.
That's a good Idea except for the dragging part, don't overcomplicate things. Like KenjiSenpai said, leave it at hovering. An even better approach would be to toggle a floating box with the item info when hovered. But then the empty area is still empty.
Do you mind if I ask why you need that area at all? Usually you put areas, boxes, whatever because you have a use for it. What you're doing is trying to find a use for an allready existing box. What's the point?
Some kind of optimal size for the window? I would say the best thing would be to just remove the upper part, since it will look empty and misplaced when neutral, and showing item information has better solutions.