If you want to change the size of individual lines of text, you will have to delve into some custom HTML. Here are some examples based on this Stackoverflow question:

---
title: "Untitled"
output: ioslides_presentation
---

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

## Font Sizes

- Standard font
- <span style="color:red">Change the font colour</span>
- <font size="1">This is some text size 1!</font>
- <font size="1">This is some text size 1!</font>
- <font size="2">This is some text size 2!</font>
- <font size="3">This is some text size 3!</font>
- <font size="4">This is some text size 4!</font>
- <font size="5">This is some text size 5!</font>

Hope that helps!

As a tip, Stackoverflow is a good place if you are looking for general guidance. GitHub issues are less monitored, and are better used if there is a suspected bug or issue with the package or software. You may also consider https://community.rstudio.com for questions.

Thanks, @mikey-harper True, I hadn't tried HTML tags yet. I was looking for Markdown-based shortcuts and skipped checking out the most obvious option!

Markdown by design only allows a limited amount of formatting. But its power comes from this simplicity :)

The cheatsheet sets out pretty much all the built-in formatting: http://www.rstudio.com/wp-content/uploads/2016/03/rmarkdown-cheatsheet-2.0.pdf .