Changing the font size of figure captions in RMarkdown pdf output

Question

I would like to make the font size of all captions (figures and tables) in my R Markdown document smaller. I'm using bookdown. The final output is pdf and I'm working in R Studio. To load the picture, I use the include_graphics function from knitr, because I've been told it's the best way (see here).

I only found the exact same question, but for html output here

An example .rmd:

---
output: pdf_document
---

Normal text has the same size as the captions.

```{r, echo = FALSE, out.width = '50%', fig.cap = "The caption has the same size as normal text."}
knitr::include_graphics('logo.png')
```

As you can see, the caption font size and the regular text font size are exactly the same, which doesn't look that nice. How can I solve this problem?

Answer 1

If it's acceptable using LaTeX packages, you can use caption

---
output: pdf_document
header-includes:
   - \usepackage{caption}
   - \captionsetup[figure]{font=scriptsize}
---

Normal text has the same size as the captions.

```{r, echo = FALSE, out.width = '50%', fig.cap = "The caption has the same size as normal text."}
knitr::include_graphics('logo.png')

```

Replace scriptsize to change size. You can find a list of default LaTeX font sizes here:

https://en.wikibooks.org/wiki/LaTeX/Fonts#Built-in_sizes

The caption package on CTAN:

https://ctan.org/pkg/caption