The suggest dimensions for a Twitter header should be the following: 1500 pixels wide by 500 pixels in height. This size will show your header correctly on most desktops and mobile devices.
- Header photo : 1500px by 500px, which has an aspect ratio of 3:1
- Photos formats: JPG, GIF, or PNG
Check out https://blog.snappa.com/twitter-header-size for full header information for Twitter
