Background Size for Twitter
Twitter allows you to share updates with family, friends and other associates using short 140-character messages known as "tweets," but this isn't the only means of expressing yourself using the service. One way to personalize your profile is to use a custom background, either from the default backgrounds that Twitter provides, or your own image. Keep in mind that in many ways, the size of the image you use is as important as the image itself.
Customizing Your Background
Twitter offers you two options for customizing the background of your Twitter page: choosing from a pre-designed background or uploading your own image. You access both using the same means: click your profile name, then "Settings, then the "Design" tab, then "Change Background Image." Select from among the several options Twitter has available for you, or click "Upload" to upload your own image file, whether it's a picture you've taken yourself, or one you've downloaded from the Internet.
File Size and Format Limits
Twitter limits the size of images you can upload as a background image. Specifically, your file cannot exceed 800 KB in size. Restrictions on image dimensions aren't as rigid, because the amount of physical space your image occupies on your screen depends on its resolution, or quality. As of 2011, Twitter only allows you to upload background images if they're formatted as JPG, PNG or GIF files, the latter of which can't be animated.
Screen and Browser Sizes
One issue you should take into consideration when choosing an image is that computer and web browsers have different display capabilities, with some able to display large, wide images, and others limited to smaller, narrower ones. For the best across-the-board results, choose a background image with common dimensions, such as 1024 x 768 or 1280 x 854. This will accommodate the majority of computer makes and models, as well as web browsers.
Another consideration you should make when choosing your background image is that your Twitter feed occupies the middle 80% or so of the page your followers visit when they view your profile. As a result, you should choose an image whose content displays not only in its center, but also around its periphery. Otherwise, other users might not be able to fully see your image.