
Ronald M. answered 12/08/20
Experienced Illustrator User
What you need to do is use Photoshop to create a file with the dimensions you need for your new thumbnail. Indicate the height and width of the pixels. Make sure you put in "72 dpi" for web display.
After that, go back to Illustrator and select your entire file and copy/paste it into Photoshop as a smart object. I do this all the time for social media content and posting to my website.