knitr – How to resize an R plot in the report generated by compile report (Not in R Markdown)?

In R Studio, there is an option to compile a report without writing codes in the R Markdown. The keyboard combination on Mac is Shift + Command + K. When I use it on the following code:

par(mfrow = c(2,1))  

plot(pressure$pressure~pressure$temperature) 
curve((0.168 + 0.007*x)^(20/3), from = 0, to = 400, add = TRUE) 
mtext("Temperature vs. Pressure (Unadjusted)", side = 3, cex = 1.2) 

plot(pressure$pressure^(3/20)~pressure$temperature) 
abline(0.168,0.007) 
mtext("Temperature vs. Pressure (Adjusted)", side = 3, cex = 1.2) 

it gives a report like this:

Is there a way to resize the plot in the report? For example, how to set it to width 800, and height 1000 as in the following image?