Code chunk font size in Rmarkdown with knitr and latex

Picking up the idea to alter a knitr hook we can do the following:

def.chunk.hook  <- knitr::knit_hooks$get("chunk")
knitr::knit_hooks$set(chunk = function(x, options) {
  x <- def.chunk.hook(x, options)
  ifelse(options$size != "normalsize", paste0("\n \\", options$size,"\n\n", x, "\n\n \\normalsize"), x)
})

This snippet modifies the default chunk hook. It simply checks if the chunk option size is not equal to its default (normalsize) and if so, prepends the value of options$size to the output of the code chunk (including the source!) and appends \\normalsize in order to switch back.

So if you would add size="tiny" to a chunk, then all the output generated by this chunk will be printed that way.

All you have to do is to include this snippet at the beginning of your document.

\tiny

```{r}
summary(mtcars)
```
\normalsize

available options for size in descending order are:
Huge > huge > LARGE > Large > large > normalsize > small > footnotesize > scriptsize > tiny