Since today Twitter allows for a new cover photo to be uploaded.
Twitter recommends 1252×626 pixels and somewhere else also 1500x500p pixels, but when I upload an image of that size, only a part (top) of the image is shown, so some unwanted resizing seems to be done. I also checked this site: http://twitterheadersize.com/, but also the 520×260 recommended by that site does not show the full image in the header.
All I want to upload an image to Twitter and see that exact image as the coverphoto, but somehow that seems to be impossible.
Best Answer
Here is good information about the size of images.
https://dev.twitter.com/docs/user-profile-images-and-banners
My recomendation is that you should design the 1500 x 500 image with a bleed area, i mean, test what portion of your design is croped for twitter and be sure that your more important content doesn´t get croped.