Tablet devices can have screens larger than 768 pixels but the pixels are dense (high dpi) so the circles drawn at 10 pixels and are small and hard to touch.
I did some checking and it doesn’t look easy to get the DPI of the device from the browser. If there is a way to detect a "mobile" device (browser string or other framework feature…??) then you could factor that in when you determine the symbol size. I would rather error on making the circles too big rather than too small.
@jacksoe: