The automatically generated legends in ArcGIS often get the job done, but when a level of customization is required, the default level of customization just isn't enough. This Post will attempt to give you some general insight into how to go about customizing ArcGIS legends.
1) First, you need to make the legend box into individual editable items. You do this by right clicking on the legend and selecting "Convert to Graphics". This breaks the ArcGIS legend format into its base parts.
2) You now need to select the legend as a whole and select ungroup (from the right click menu). You may also need to select each individual legend item and click ungroup again as they are often bound together
3) Now that your ArcMap legend is unbound, you can move each element around like as if they were drawn individually by you. ie. you can use the usual editing/drawing tools on them