Recommended: 800 x 200 pixels
Free Template: https://docs.google.com/presentation/d/1YgNMPJ0TsRSgdQEJCvqgBlsXgTKJiqrDl5ZJTukFbTk/template/preview
EXTRAS:
Create an Animated Google Classroom Header Using Canva + Google Slides (FREE TEMPLATE INCLUDED!) https://www.youtube.com/watch?v=ScAymRlytl8
Quick Tutorial of Creating a Customized Header for Your Google Classroom
Recommended: 1600 x 400 pixels
Template: https://docs.google.com/presentation/d/1Bk44OSyAtyK0-Rq3LHaRBrBASFt6hBFrvzPO-sIs1Ew/template/preview
EXTRAS:
The 10 Best Google Forms Templates
https://helpdeskgeek.com/how-to/the-10-best-google-forms-templates/
How to Customize Google Forms with Themes, Images, and Fonts
https://www.howtogeek.com/542310/how-to-customize-google-forms-with-themes-images-and-fonts/
Recommended: 250 x 250 pixels
Tips:
Professionals recommend choosing a larger image for better appearance.
Note that it can accept maximum file size of 100MB for the profile pictures.
Recommended: 1080 x 608 [16:9]
Tip: You can go for as large as up to 2120 x 1192 pixels and can also pick a smaller image with a minimum size of 480 x 270 pixels.
EXTRA:
Google+ Cover Photo - Tips and Tricks
http://www.skipser.com/p/2/p/google-plus--cover-photo---tips-and-tricks.html
Recommended: 150 x 150 pixels