Better design practice is to label the bars directly rather than keeping the label information off the the side in a legend box away from the data. Edward Tufte preaches about this sort of thing constantly.
As for your specific request, it seems that if you alter the Legend.Position property, the size of the box markers in the legend scales with the height of the legend box. So, if you increase the height, the vertical size of the box markers will change. The horizontal size doesn't seem to change. In my experimenting, the legend box moved when I did this so you'll need to manually set the x and y values so that your legend box is where you want it in the figure window.