What is the default dimension for a user's Twitter's Background image?
What are the default dimensions for a Twitter User's Background image?
If I want to upload my own image for my Twitter account's Background image, what should its default dimensions be, as per Twitter's official guidelines?
If you are talking about the Cover image, here is a good site with this information:
What is the new twitter header resolution?
Twitter recommends 1200Ã—600, but it will resize any uploaded image to 520Ã—260 pixels.
If you are talking about the whole page, user background, here is a good tutorial. Recommended size on there is 1600px by 1200px. (width by height)