Hi @artgirl thanks for the question ;) The main banners would be for: Twitter / FB / YT / Reddit. The ideal sizes often change along with layout changes. Here's a useful link i found (2017).

https://louisem.com/2852/social-media-cheat-sheet-sizes

An ideal Twitter banner, for example, is 1500 pixels wide by 500 pixels tall (2018).

I believe these are the ideal header/banner sizes for:

FB - 828 wide x 350 tall

Youtube - 2560 wide x 1440 tall