There is no one size requirement and the image will be scaled depending on the browser size.
On mobile, there's a minimum height of 100px and maximum height of 270px. The width may vary, but it's around 320px.
On desktop, there's a minimum height of 100px and maximum height of 460px. The width may vary, but the maximum is 620px.
In order to see changes reflected in your profile, refresh your browser page.
You may encounter some problem where you are unable to upload your photo. Make sure that the image's filename does not include any special characters and that the file size does not exceed 2MB.
You can also try clearing your browser's cache and cookies or try using a different browser.