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?

You can set the figure width and height using special comments (similar to chunk options) e.g.

#+ fig.height = 10.4, fig.width = 8.3

above where you create the plot. Note that here the figure sizes are always given in inches (not pixels as in your example, so you would need to convert from pixels to inches).