You could convert your legend to graphics and then play around with your symbols (you'd need to ungroup things a couple of times before you can select individual symbols.)
BUT you'll loose the connection between legend and map, so changing anything in your map (like adding layers or changing symbols) will no longer be reflected in the legend.
From a cartography point of view I think symbols should always be the same size on the map and in the legend, especially if you use several symbol sizes (like small dots for one category and bigger dots for another). If you want to save space, you might want to think about using the shortest possible names for your layers, using less space between the symbols or leaving off layers that are self-explanatory. E.g. if you have a lake on your map that's labelled as Lake such-and-such, you don't need to symbolize the lake in the legend.
Hope this helps,
Kathi
