You can't modify the legends size only format the title and body.
You can play with shapes in the marks card and also create your own shapes.
You can place your shapes in \My Tableau Repository\Shapes folder.
Close and reopen tableau desktop then go to shapes and reload shapes. Use those shapes in your worksheets.
I am having almost 12 charts on the dashboard. Considering that it will be very tough to create a separate shape chart for everyone and create the highlight action for all of them. Solution will surely work if there are less number of charts, but in case more number of charts there will be more number of sheets and actions to be managed.
Thanks for the reply.