How to add more font sizes to existing font sets

Hi,
You can try with the following code:

.single-detail .entry-title { font-size: 20px;}
.single-detail .main-content { font-size: 18px;}
.block .block-title { font-size: 22px;}

Also, you can open the teplate.css file in the folder path “wp-content\themes\dw-fixel\assets\css”, then find the change the size in the line code.
Hope this helps !