Ok, so time to look under the hood a bit at the monospace theme css. First step will be to just open the page in chrome, and then inspect element.

So, the blog title just inherits the default font size of 38.5px from the bootstrap.min.css. This is also the same in the unminified bootstrap.css. This is way too big. However, I see it's given an ID of "blog-title". This may be handy. Now let's look at the post titles. I'm guessing this is the same thing.

And indeed it is. But, no ID. Monospace has it's own css in /themes/monospace/assets/css called "theme.css". Thankfully it's very short and very readable. Easy enough to add a "font-size: 22px" to the blog-title id. Thus, this:

#blog-title { margin-top: 0; line-height:48px;}

becomes this:

#blog-title { margin-top: 0; line-height:48px; font-size: 22px;}