How to Change Footer Height in 3.4.2 TET

In your site, set a height on #site-generator in your child style.css.

Please explain in details how and what should I change in #site-generator. I have already # site-generator in the child theme.

Yes, I know — that’s why I said to use it. You need to add a height attribute:

#site-generator {
   height: XXXpx;
}

This site may be useful in learning some basic CSS:
https://www.w3schools.com/css/