For one thing, you might find the “Modern” format a bit better to work from as a starting point, since it’s designed for standard full-page printing, whereas the “Paperback” formats have more stuff to “undo”, if you will, like odd paper size, alternating margins on odd/even pages, etc.
But either way, there is no global font size override, as that would rarely be of any use to anyone, given body text, headings, page footer/headers, footnotes and such all tend to be different. So you may have to do a little more customisation than you would for the font family, which is easily done with the font override above the preview column in the main compile overview screen.
So speaking of that area, if your formatting source is fairly simple, you may only need to adjust the Titled Section layout. Double-click on it in the preview column to open the format designer, and jump straight to it.
In the Formatting tab, click into the main body text in the example, and change the font size. You may also want to boost the heading size to keep it relatively larger.
Yup, that should be about it. 
Like I say though, it may require more customisation if you have a fairly complex document in terms of formatting. You may also need to go into the Styles pane in the sidebar here, and go through any of those styles you make use of and boost the font size for each as well—or at least make sure the Include font size checkbox is disabled per style, should it not matter much to that style’s intent.
If that’s not good enough and you need more, the previous how-to linked above should help with more comprehensive changes.