What matters
X Profile Photo dimensions at a glance
X profile photos display as circles everywhere — next to tweets, in notifications, in follower lists. A 400×400 source gives enough resolution for all display sizes.
Your avatar is your visual identity in every conversation. At 400×400 you get sharp rendering even on high-DPI screens without an oversized file.
Non-square uploads get force-cropped into a circle. Low-resolution images look pixelated in larger display contexts like profile pages.