Code chunk font size in Beamer with knitr and latex

Question

I am trying get some R code to fit on my beamer slides. It does not seem possible to change the font size via the size argument for the code chunk as you might do for other knitr type documents. The only way seems to be with \footnotesize before every code chunk. This gets frustrating, as I have lots of code chunks and in many cases I then have to use \normalsize after for my LaTeX bullet points.

---
title: "Untitled"
output:
 beamer_presentation:
  includes:
   in_header: header.txt
---

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

## R Markdown

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

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

In my header.txt (below) I have experimented with a couple of bits of code from http://yihui.name/knitr/demo/beamer/ but with no luck.

\ifdefined\knitrout
\renewenvironment{knitrout}{\begin{footnotesize}}{\end{footnotesize}}
\else
\fi

\makeatletter
\let\oldalltt\alltt
\def\alltt{\@ifnextchar[\alltt@i \alltt@ii}
\def\alltt@i[#1]{\oldalltt[#1]\footnotesize}
\def\alltt@ii{\oldalltt\footnotesize}
\makeatother

... but really out my depth with \def.

Answer 1

Following @Martin Schmelzer's output, you can change code font size and text font size independently and for the whole document by adding this to you rmarkdown file:

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 \\normalsize")
})

Out of this code snippet, you only have to change the "footnotesize" and "normalsize" parameters to whatever font size you want; the first being the code and output font size and the second being the text font size.

For instance, with code as "tiny" and text as "normalsize":

---
output: beamer_presentation
---

```{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 \\", "tiny","\n\n", x, "\n\n \\normalsize")
})
```

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

# Section 2
```{r}
summary(cars)
```
This works for every chunks.

Gives this:

Answer 2

Drawing on this tex.SE answer, we could redefine the Shaded environment that surrounds R code to make it footnotesize (and the verbatim environment for output). Add this to your header.txt:

%% change fontsize of R code
\let\oldShaded\Shaded
\let\endoldShaded\endShaded
\renewenvironment{Shaded}{\footnotesize\oldShaded}{\endoldShaded}

%% change fontsize of output
\let\oldverbatim\verbatim
\let\endoldverbatim\endverbatim
\renewenvironment{verbatim}{\footnotesize\oldverbatim}{\endoldverbatim}