A thumbnail is seen at about the size of a postage stamp before anyone decides whether to click. Everything about making one follows from that.
The mistake everyone makes is judging a thumbnail at full size. Scale your finished image down to about two hundred pixels wide and look at it there. If you cannot read the text or tell what the picture is, neither can anyone scrolling.
Three or four words is the ceiling. The title is already next to your thumbnail, so repeating it wastes the space. Text on a thumbnail should add something the title does not.
Faces and high contrast do well because they survive being tiny. Fine detail, small text and busy backgrounds do not.
1280 by 720 pixels is the standard, and any 16 by 9 image scales to it cleanly.
Most platforms cap thumbnails at a couple of megabytes. Compress if yours is larger.
No.