As we continue to roll-out the new Twitter profile pages (https://blog.twitter.com/2014/coming-soon-a-whole-new-you-in-your-twitter-profile), we now have updated guidelines for image sizes that you upload via the REST API. We now recommend that profile banners be 1500x500 pixels in size, and that profile images (avatars) be 400x400 pixels in size. We’ve updated our documentation around this on the following pages.
Profile Images (400x400):
Profile Banners (1500x500):