Code chunk font size in Rmarkdown with knitr and latex

In knitr, the size option works fine in a .Rnw file, the following code generates:

\documentclass{article}

\begin{document}

<<chunk1, size="huge">>=
summary(mtcars)
@


\end{document}

However, I can't get it to work in Rmarkdown. The following code does not change the font size, as it did in .rnw file. The same thing happens when trying to set options with opts_chunk$set(size="huge").

Is this the expected behavior? How does one change the chunk code font size? (I mean using knitr options, not by adding \huge before the code)

---
title: "Untitled"
output: pdf_document
---

```{r, size="huge"}
summary(mtcars)
```

I am using RStudio Version 0.98.987, knitr 1.6 and rmarkdown 0.2.68.

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

Per this Gist, you have to define the font size using css:

<style type="text/css">
body, td {
   font-size: 14px;
}
code.r{
  font-size: 20px;
}
pre {
  font-size: 20px
}
</style>

code.r will control the font size for R code echoed from the code chunk, while pre will apply to any R results output from the code.

A complete working .Rmd file might look like:

---
title: "FontTest"
author: "Thomas Hopper"
date: "January 13,2016"
output: html_document
---

<style type="text/css">

body, td {
   font-size: 14px;
}
code.r{
  font-size: 20px;
}
pre {
  font-size: 20px
}
</style>

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:

```{r cars}
summary(cars)
```

The resulting html renders as:

You can define you own document format by exporting something based on the following function from your package my_package:

my_report <- function(...) {

  fmt <- rmarkdown::pdf_document(...)

  fmt$knitr$knit_hooks$size = function(before, options, envir) {
    if (before) return(paste0("\n \\", options$size, "\n\n"))
    else return("\n\n \\normalsize \n")
  }

  return(fmt)
}

This will define a knitr chunk hook size that will put the appropriate latex command before the chunk, and \normalsize after the chunk.

Anyway, with the following R markdown you can check if it's working:

---
output: my_package::my_report
---

Test text for comparison

```{r}
print(1)
```
The next code chunk has `size = 'tiny'` in the chunk options.

```{r, size = 'tiny'}
print(1)
```

I get the following result from `markdown::render():

See also the issue I opened on github:

https://github.com/yihui/knitr/issues/1296

Following up on @Martin Schmelzer's output, here's a solution in the case you want to change the code and output default font size for the whole document, but not the size of the text.

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

For instance,

---
output: pdf_document
---

```{r setup, include=FALSE}
def.chunk.hook  <- knitr::knit_hooks$get("chunk")
knitr::knit_hooks$set(chunk = function(x, options) {
  x <- def.chunk.hook(x, options)
  paste0("\n \\", "footnotesize","\n\n", x, "\n\n \\Huge")
})
```

# Section 1
```{r}
summary(cars)
```
Text.

# Section 2
```{r}
print(1)
```
This works for every chunks.

gives this: