I added a 600 x 600 (pixel) square icon as the Social Sharing icon for my website. Each social media platform is different in the way it shows it. Twitter shows a square of the full icon. LinkedIn and Facebook cropped the images as different-sized rectangles. You'll want to keep the most important part of your icon in the center of your icon image. I also adjusted the central focal point on my original icon image to be close to the flower rather than the center of the image. This helps to keep the most important parts of my icon, even if cropping occurs.
These were the actual image sizes on my Macbook Pro:
Twitter (250 x 250 pixels)
LinkedIn (380 x 200 pixels)
Facebook (950 x 500 pixels)
(From KerryAThompson.com Blog: Best practices for sizing your Social Sharing icon)