The Facebook Profile Picture size (170×170px)
Facebook displays profile pictures at 170x170 pixels on desktop in a 1:1 square, shown as a circle. This avatar sits on your profile, overlapping your cover photo, and appears beside every post and comment you make, so a clean square upload at this resolution keeps it looking sharp rather than pixelated.
Facebook crops the picture into a circle, so keep your face or logo centered with a little padding to avoid clipping the corners. Note that while it displays as a circle in most places, it can appear as a square thumbnail in some spots like tagged photos, so a centered subject is the safest choice for both crops.
Because the avatar renders small next to posts and in the friends list, a well-lit face or a bold, simple mark reads best; fine detail and small text vanish at that scale. Uploading a crisp 170x170 gives Facebook a clean source, though Facebook may store a larger version, so a subject that stays clear when shrunk is what matters most.