Hello @ayham,
Thanks for updating the thread. 
If you had used Cornerstone to create the blog posts then you have the option to style the font sizes. I see that you are not using Cornerstone to create blog post, so in your case you will need to make the changes in blog pages using custom CSS.
Code that @Rad shared seems to be working fine that changes font size of excerpts. If you would like to change the font size blog inner post, please add following CSS under X > Theme Options > CSS:
.single-post p {
font-size: 18px !important;
}
You can change the values as per requirement.
If you would like to learn CSS, I am sharing few resources that you take a look to get started with CSS and an interesting tool that you can use to speed up the development process.
I recommend you to watch following video that will help you to get started with CSS.
https://www.youtube.com/watch?v=MFR4WXiLzpc
Sometimes it can get a bit difficult to find out the right selector to be able to write the required CSS codes. A handy tool that can help you in this is Google Chrome dev tools. I am sharing the resource that you can refer to get started with dev tools.
https://developers.google.com/web/tools/chrome-devtools/css/
https://developers.google.com/web/tools/chrome-devtools/
https://www.youtube.com/watch?v=tP_kXBJWPhQ&t=200s
Thanks.