It depends on what you want to do with the plot. Most often, I'm exporting as a png to use elsewhere. So I mostly use ggsave("mygraph.png", height = , width =, units = "cm", bg = "white") and adjust height and width to suit the plot. Often the text and labelling sorts itself out in this case.
You can change the plot size in markdown for the whole file, or in the heading for each block. Width and aspect ratio are the things to look for, I believe.
Ok a guy on Slack gave me the solution and it worked
"set the image size in the arguments to the code chunk (look for the little gear just to the left of the green triangle
and you can use scale_y_continuous() to set the breaks= and labels= arguments."