You can change the font size in R Markdown with HTML code tags <font size="1"> your text </font>
. This code is added to the R Markdown document and will alter the output of the HTML output.
For example:
<font size="1"> This is my text number1</font> _x000D_
_x000D_
<font size="2"> This is my text number 2 </font>_x000D_
_x000D_
<font size="3"> This is my text number 3</font> _x000D_
_x000D_
<font size="4"> This is my text number 4</font> _x000D_
_x000D_
<font size="5"> This is my text number 5</font> _x000D_
_x000D_
<font size="6"> This is my text number 6</font>
_x000D_