The Twitter / X Profile Picture size (400×400px)
Twitter, now X, uses a 400x400 pixel square for profile pictures, displayed as a circle. Uploading a clean 1:1 image at this size keeps your avatar sharp across the timeline, replies, and your profile page, avoiding the fuzziness that comes from letting the platform upscale a smaller file.
Since X masks the square into a circle, the four corners are cropped away. A logo or face pushed into a corner will be clipped, so center your subject and imagine a circular boundary when you compose. This matters most because the avatar appears tiny next to every post you make and reply to.
At timeline scale the avatar is only a few dozen pixels wide, so intricate detail turns to mush. The strongest profile pictures use one bold, high-contrast element, a single face, a clear monogram, or a simple logo, that stays recognizable when shrunk. Preview it small before you commit, because instant recognition is the entire job of this image.