Finding helpful social media templates seems to be a chore, so I have curated the dimensions for various items across social media platforms and put together templates for you to use.
Header Image (1500x500, 1263x421)
Many sites list the header image to be 1500 pixels wide by 500 pixels high, however they also mention that you can make this 1263 pixels wide by 421 pixels high if you wish to not have your header image resized. This first set of dimensions is what I personally use when creating Twitter header images for my accounts.
Download Layered SVG (Inkscape)
Avatar (500x500 square)
While I say square, keep in mind that most social platforms including Twitter have switched to showing your avatar as a circle. So keep that in mind when creating an avatar, especially when they include text.
Youtube
Channel Art
YouTube Channel art has a max size of 2560 x 1440 pixels and a maximum file size of 6MBs. This is the very template I used to create my own YouTube channel art.
Download Layered SVG (Inkscape)