rMarkdown: change fontsize for Beamer

Question

I am moving my workflow to R/RMarkdown that outputs Beamer slides. I give about 60 presentations per year, and I'm loving the automated flow. However, I need to increase the base font size in my Beamer output. Using the code below, the size of the bullet-pointed text is indifferent to the value in fontsize. I can input 12pt or 24pt, and the option changes in the .tex file, but there is no change in the resulting Beamer pdf. I'm compiling from within RStudio in win10.

title: "Title"
author: "Matthew C. Roberts"
date: "October 2016"
fontsize: 12pt
output: 
    beamer_presentation:
---

## Blah

* Hi, my name is Matt
* This is a bullet point
    + so is this
    + this is fontsize

Answer 1

As @samcarter_is_at_topanswers.xyz writes in his comment, it is correct to use the option fontsize; but you just cannot use arbitrary numbers. At the moment, only the numbers 8pt, 9pt, 10pt, 11pt, 12pt, 14pt, 17pt and 20pt work (see the documentation beameruserguide.pdf that comes with beamer, Section 18.2 "Font Changes Made Without Using Font Themes"). So you should check whether 20pt are enough.

Another strategy is to fiddle with the paper geometry. The default is that slides are of format 128mm by 96mm. If you specify smaller values, then, obviously, the font size will appear larger in full screen mode. However, beameruserguide.pdf advises against this strategy (in Section 8.3 "Margins and Sizes").