Most icons are also circular so as to take uo most of the available space. R and S together in a circular fashion instead of a rectangle would be ideal.
I believe the best way is to use rectangle, not a circle, if you would want to make the most of available space in 16x16 pixels rectangular frame
The problem with rectangular is that you end up leaving whitespace on top like the current logo.
Most icons on coinmarketcap appear circular but a large RS in a 64px block would work too (but maybe not look as nice).