Icons are 512×512 PNG images, rule is simple and tested by me from beggining. Works great and breaking this rule will always damage your installs. You are not brand, you have not millions of dollars to promote your cool symbol icon like McDonalds, Avast, Supercell, etc. Do not try to make icon with symbol, cool logo, etc. People will forget it in next 5 minutes… instead of that use keyword on icon like here, protips, some icons were stolen from me by other devs:

App Icon

It’s simple, you need to write big keyword on your icon to bring attention of users who are looking for feature.
Of course, you need to name your icon with keyword too.