Find a copy of one of Google map icons - this probably will be .png type:
Get it onto your system.
Make sure the file name follows Android Studio's rules - ie. no hyphens.
Open the file in your favourite paint program and change it to the color you want and save it without loosing its transparency. GIMP is a good for this.
In Android Studio, right-click on res/drawable folder.
Click on Show in Explorer.
Go into drawable folder.
Copy/paste your new icon into res/drawable folder - don't worry about it being a .png file while the others in the folder are .xml files.
Close Explorer window.
To use in your app - assuming the file name is gray_dot.png:
mMarkerOptions.position(beachCoord).title("marker title").snippet("marker info").icon(BitmapDescriptorFactory.fromResource(R.drawable.gray_dot));
When you are typing R.drawable.gray.. - it shows up without the .png extension.
Run your app.
Basic testing shows that the icon size works on a tablet and phone just fine without worrying about the different resolutions. I will do more testing to see if this remains true for higher resolutions.