When using a word processor/text editor, you will often change the size of your type, for various reasons by setting a numeric size option. You can achieve the same results when creating an R Markdown document using some inline LaTeX code. I’ve written before about a setting the font size in your document by using settings within your YAML header, but noted that you could only use one of three options (10pt, 11pt or 12pt) in PDF output. This limitation is due to standard latex classes only accepting those three.
To overcome this, you can use inline LaTeX code in your R Markdown document to specify relative sizes for your text. Instead of using 12pt or 14 pt, you use Huge or Large or other variations to produce different sizes of text. Here is the output from an example R Markdown document that shows the different sizes: