try pressing the zoom button in the plots subwindow of Rstudio.
You could resize the plot with your mouse and see how it looks at different sizes.
Also, you could save the plot to an image of a fixed size, and then view it in the normal way you would view an image file.
data(iris)
png("irisexample.png",width=15,height=15,units="cm",res=200)
plot(iris$Sepal.Length, iris$Sepal.Width, col = iris$Species)
legend("topright", legend = levels(iris$Species), col = 1:3, pch = 21)
dev.off()