The best way would be to be able to control the size in Rstudio, but as you have found out yourself from the Rstudio-website, Rstudio doesn't support that. The following code saves your plot to wmf. There is also a workaround to a save to bitmap, which involves some clicking, but at least you don't have to specify the size any more:
data(mtcars)
windows(800, 600, pointsize = 12) #opens a separate window with the size you want
hist(mtcars$mpg) #draw to this (active) window
savePlot("clipboard", type="wmf") #saves plot to WMF
Unfortunately, it seems to be impossible to save to jpg
format to the clipboard. You can copy it to a bitmap by going to this window, click CTRL-C
and the graph is on the clipboard as bitmap with 800:600.
EDIT:
The windows
command only works on Windows.
For Mac, it should be replaced by: quartz(width=8,height=6,pointsize=12,dpi=100)
(width/height in inches!)
For linux try x11(width=8,height=6,pointsize=12,dpi=100)
(untested).