Follow below steps to change blogger posts font size 

step 1 : go to blogger dashboard
step 2 : click on template
step 3 : click on edit HTML
step 4 : search      .post-body {  
add below code before closing tag of above line
font-size: 16px;
step 5 : click on save template.

Now your blog post text size is 16. Like this you can easily change the font size of your blogger blog posts. Another method is also there to change blogger blog post font size.
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