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.
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.
Chris Donohue, GISP
How long will it take to get this functionality up and running?? They want us to move forward with these new programs, why can't they at least be consistent in something I would think is so easy? I want my stream on my map to look like a stream in the legend, not a straight line. I hope this is updated soon!
Hi Kirstyn! I had the same question a while back, and here's what ESRI told me: "The legend patch shape? That functionality is not available in Pro yet. The shape in the preview when you are editing symbols/symbology is only for that preview window in that pane and has not bearing on the legend."
(link to my thread with some additional info/discussion: How does one change the symbol patch in a legend? (Pro 1.2) )
Here's a clutter creating workaround: Create a dummy layer to be used only in the legend that is a copy of the legend item with the patch you want to change. Use that layer in the legend, adjust its symbology width as desired and turn it off in your map but keep it in the legend.
Hi Chris Schrader, follow the steps on this web help page, and let me know if you have further questions.
So it's done through the contents pane, rather than in the legend itself?
That seems a bit absurd.
Changing the legend patch is something you generally do for the end user of a map; someone who needs a squiggly line in the legend to help understand that it's for a stream. That's what a map legend is for.
The desktop user who is setting up the map already knows which features are streams. I don't thing having it show up in the contents pane is particularly useful.