Let try this minimal Rmarkdown file
---
title: "cex in Rmarkdown"
output: word_document
---
```{r}
knitr::opts_chunk$set(dev='pdf')
```
```{r}
plot((0:160)/4, 0:160, type="n")
text(x=20, y=70, labels =expression(alpha), cex=1e-7)
```
When knitr-red from Rstudio (with r 3.6.1 on MacosX with knitr 1.24), it
produced an error with this message ("Information of metric non
available for this family") [translation from French because of my
system configuration]
However,
text(x=20, y=70, labels =expression(alpha), cex=0)
and
text(x=20, y=70, labels =expression(alpha), cex=0.1)
work.
Also text(x=20, y=70, labels ="a", cex=1e-7) works.
The error does not occur when dev='png' is used but it occurs also for
dev='postscript'
I don't know where I should report the bug.
Thanks