What is the Twitter Image Background Size
The profile picture display is 73 x 73 pixels, but you can upload an picture up to 500 x 500 pixels.
Header Image: 520 x 260 px Twitter’s recommends header size is 1252 × 626 pixels.
The maximum file size for header images in Twitter is 5MB.
The largest size (1820 x 1080),
Background Size Limit, Twitter says the maximum file size for background images is 2MB.