The size of the plot is dependent on the graphic device so depending on what the format of your final output is going to be you could for example change the size while saving the file
ggsave(g, height = ..., width = ...)
or define it on the code chunk if you are using an .Rmd file
```{r, fig.height=..., fig.width=...}
# Ploting code
```
For example:
---
title: "R Notebook"
output: html_notebook
---
```{r, fig.height=2, fig.width=10}
library(tidyverse)
df <- tibble(
brand = c("Facebook", "Google", "Twitter"),
market_share = c(.4,.5,.1)
)
ggplot(df) +
aes(x = "Total", y = market_share, fill = brand) +
geom_col(width = 0.5) +
labs(title = "My Title!") +
coord_flip() +
theme_void() +
theme(plot.margin = margin(30, 10, 0, 10))
```