Font size support

_(Extracted from #149.)_

Il standard LaTeX, there's [a couple of standard font sizes](https://en.wikibooks.org/wiki/LaTeX/Fonts#Sizing_text):

- `\tiny`
- `\scriptsize`
- `\footnotesize`
- `\small`
- `\normalsize`
- `\large`
- `\Large`
- `\LARGE`
- `\huge`
- `\Huge`

Let's start implementing sizing from those (and probably make it configurable?)

There's also the base `fontsize` command that accepts some arbitrary units of measure. We'll have to think more about how to implement them, and which units we want to support.