I don’t have an official list, but did do some research.

https://mt.google.com/vt/icon/text=A&psize=16&font=fonts/arialuni_t.ttf&color=ff330000&name=icons/spotlight/spotlight-waypoint-b.png&ax=44&ay=48&scale=1

Use that URL and you can easily change the text, size,font and color to your preference,
however the color parameter will only change the color of the text.
but if you change waypoint-b.png to waypoint-a.png you get a green marker.

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Icon list:

Markers:

Places:

   



   



   



   



   

   



   



   



   



   

   



   



   



   



   

   



   



   



   



   

   



   



   



   



   

   



   



   



   



   

   



   



   



   



   

   



   



   



   

   



   



   



   

   



   



   



   

   



   



   



   

Traffic:

Miscellaneous:

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Look at the URLs and notice the L which can also be S or M, notice the _v_ and _search_ in the names.
At the transit icons you can change large to tiny, mini, and small.

There is probably more. but thats all I have found so far.