I actually ran into this some time back and sad to say at this time you have no means of affecting the font size in default visualizations. This is because the data inside these sits inside a canvas which is, for all intents and purposes, a container displaying info from an outside source.
This holds true for label colors and font families as well.
is there any work around availble because users are really frustrated with the these since they are using different screen resolutions and we are building dashboards in different resolutions. and we are not able to ask each users to change them
we are facing this issue when user open the dashboard itself and they are not happy with the visualisation since ledends are hidden. also we are not able to ask each user to do the above steps to see the legends