The KakaoTalk Profile Picture size (640×640px)
A KakaoTalk profile picture works best as a 640x640 pixel square in a 1:1 ratio. KakaoTalk shows your profile image in several places, as a full-screen background when a friend opens your profile, as a large square on the profile card, and as a small circle or rounded thumbnail next to your messages in chat, so a clean square master scales gracefully into all of those shapes.
The key thing to plan for is how differently the same image is displayed. In the chat list it becomes a small circle, roughly the size of a coin, where only the center is visible, while on the profile view it fills a much larger area with your name overlaid. Keeping your face or subject centered means the circular crop in chat still shows the important part, and it prevents the common problem of a subject drifting to an edge and getting clipped out of the thumbnail entirely.
Sizing to a true 640x640 square avoids the stretching or awkward cropping that happens when a portrait or landscape photo is forced into KakaoTalk's square and circular slots. Because your profile picture is your identity across every chat, a sharp, well-centered image makes you easy to recognize at a glance. This tool resizes 100% in your browser with no upload, it is free, and it preserves the 1:1 aspect ratio. KakaoTalk occasionally changes how it crops and displays profile images, so verify the current behavior before you settle on a photo.