Circle Approximation
Circle are no more than infinite angles of a shape. To our eyes, this is hard to tell it's not a perfect circle. There are 24 straight edges
Based on this gif https://24.media.tumblr.com/3da5603e801439728ccb0a795ec7b740/tumblrmxutpjkK8g1sso6sco1400.gif by Daily Cube http://dailycube.tumblr.com/
You could change the dimensions by using find and replace for 300px or by replacing the three 300px with your desired dimension
I wish box-shadows had percents...