At least from the ESRI Help, it appears that unfortunately this is not an option. It looks like one can only do sizing of the current patch, not change its shape.
From the Help:
On the Design tab, you can set the default legend patch size. In the Settings group, you can select or input a size for Patch Width and Patch Height.
Work with a legend—Layouts | ArcGIS for Desktop
Also, it appears a potential work-around of importing a different patch shape from an existing mxd set up in ArcGIS Desktop is also out:
Because the functionality of legends in ArcGIS Pro does not match the functionality available in ArcMap, legends will be converted to use settings available in ArcGIS Pro.
Import an existing legend—Layouts | ArcGIS for Desktop
Chris Donohue, GISP