Increase the Font Size in Blog Post.If your blogger or WordPress post font size is small by default and you want to change or edit the font size for every blogger post body automatically then here is a post to increase default font size on a blog post.To change the font size of your blog posts go to Template tab and click on Edit HTML and then select Proceed.you will see your blogger template code.
 

In your blogger template HTML code search (Ctrl+F) for ".post {"

you will find a piece of code like this

.post {
margin:.5em 0 1.5em;
border-bottom:1px solid #cccccc;
padding-bottom:1.5em;
text-align: justify;
font-size: 18px;
font-family:'Segoe UI', Arial;
line-height: 21px;
}

Here (font-size: 18px;) you can change the font  to your desired size and click on Save Template.You can also change the font family by specifying the font family name in 'font-family:'Segoe UI', Arial;'and also can increase the height of the lines in blog post from the above piece of code.