@maddy6 This is the only solution to increase the size of legend markers.
If you inspect the legend attributes running the following line of code: help(go.layout.Legend) you’ll see that the only property referring to the marker size is: legend_itemsizing.
legend_itemsizing='trace' (the default value) displays the marker size proportional to the trace marker_size (as in the image I posted above)
legend_itemsizing='constant' displays both markers of the same size no matter how the marker_size was set in trace definition. Usually it displays marker size equal to the largest marker_size among all traces.