Currently (2015) if you are using templates provided by Blogger, you can only change the blog description font color.
There is still no readily available function in the by Blogger template, where you can change the blog description font type and size.
You need to do tweak the code by yourself.
This is a step-by-step tutorial on how to do it.
Change Blogger Blog Description Font Type And Size
1. Log in to Blogger Dashboard.
2. Click on "Theme".
3. Click on "Edit HTML" button.
4. Now search for this this line of code:
<p class='description'><span><data:description/></span></p>
5. Delete away this line of code and replace it with this:
<p class='description' style='font-family: verdana; font-size: 16px;'><span><data:description/></span></p>
Note: The above code is just an example. The two places which you may want to change to your preference are highlighted in white color. You need to change the word "verdana" to other font type of your choice.
You also need to change the 16px to other size to your requirement.
6. After you have done that, click the "Preview template" to check whether it is working correctly.
7. Once you are satisfied with it, then click the orange color "Save theme" button.
Related Tips:
* How To Change Blogger Blog Description Font Size with CSS.
* Change Font Size And Alignment Text Below Sidebar Widget Title With CSS For Blogger