What size is the Facebook header? Or cover photo? How big should be my profile picture?
Your Page’s profile picture:
Displays at 170×170 pixels on your Page on computers, 128×128 pixels on smartphones and 36×36 pixels on most feature phones.
Your Page’s cover photo:
Displays at 820 pixels wide by 312 pixels tall on your Page on computers and 640 pixels wide by 360 pixels tall on smartphones.
Must be at least 400 pixels wide and 150 pixels tall.
Loads fastest as an sRGB JPG file that’s 851 pixels wide, 315 pixels tall and less than 100 kilobytes.
For profile pictures and cover photos with your logo or text, you may get a better result by using a PNG file.
Keep in mind that your Page’s profile picture will be cropped to a circular shape in ads and posts, but will remain the same square shape when people visit your Page.