Hi Konstantinos,
Unfortunately, we do not yet have an easy way to resize all legend elements simultaneously while maintaining all the dynamic function of the legend. This is something that ArcGIS Pro users have identified on the ideas site - Scale Legend Contents Automatically with change in size.
If you are not worried about having a dynamic legend that updates automatically, for example when you add or remove layers or change a layer's rendering, you can convert the legend to graphics (right click the legend item in the Contents Pane and click Convert to Graphics from the context menu) and resize the newly created graphics group.
If you do need the legend to be dynamic please refer to the Legend Arrangement section of the help topic Work with a legend—Layouts | ArcGIS Desktop.
Hope this helps,
Tom
