You can successively add elements to a ggplot object. Here's an example with the built-in iris data frame.
Create a ggplot and store in an object called p
p = ggplot(iris, aes(Species, Petal.Width)) +
geom_point()
Make x-axis labels vertical:
p = p + theme(axis.text.x=element_text(angle=-90, vjust=0.5, hjust=0))
Display the plot:
p
To save the plot at a given physical size: Here's an example of saving a plot as a PDF file with width=5" and height = 6":
pdf("myplot.pdf", 5, 6)
p
dev.off()