Figure sizes are specified in inches and can be included as a global option of the document output format. For example:
---
title: "My Document"
output:
html_document:
fig_width: 6
fig_height: 4
---
And the plot's size in the graphic device can be increased at the chunk level:
```{r, fig.width=14, fig.height=12} #Expand the plot width to 14 inches
ggplot(aes(x=mycolumn1, y=mycolumn2)) + #specify the x and y aesthetic
geom_line(size=2) + #makes the line thicker
theme_grey(base_size = 25) #increases the size of the font
```
You can also use the out.width and out.height arguments to directly define the size of the plot in the output file:
```{r, out.width="200px", out.height="200px"} #Expand the plot width to 200 pixels
ggplot(aes(x=mycolumn1, y=mycolumn2)) + #specify the x and y aesthetic
geom_line(size=2) + #makes the line thicker
theme_grey(base_size = 25) #increases the size of the font
```