Header and profile image sizes vary by social media channel. The larger image sizes are better with most allowing you to make frame repositions.
In pixels:
Facebook. Header 851 X 315
Profile 180 X 180
Twitter: Header 1500 X 500 px
Profile 400 X 400 px.
LinkedIn: Standard Logo 100 X 60
Square Logo 50 X 50
Google +. Header 1,080 X 608
Profile 250 X 50
Pinterest. Profile 165 X 165
YouTube. Channel Cover 2560 X 1440
Instagram. Profile 110 X 110
Tumblr. Profile 128 X 128