One of the most important things on your blog or website should be your profile picture as it is the only thing on there that people can relate to you by and also will count as there first impression of you and like every one knows first impressions matter the most. In fact just by simply having a profile picture in the first place your blog is 14x more likely to be looked at So just the fact alone is enough for it to matter.
Facebook (180 x 180),
Twitter (400 x 400)
Google+ (250 x 250)
Linked in (100 x 60 horizontal or 50 x 50 square)
Instagram (110 x 110) mobile, (180 x 180) desktop
For more great social media info look at my twitter stream DirectorZwarden