Here is a my solution to this:
resize.win <- function(Width=6, Height=6)
{
# works for windows
dev.off(); # dev.new(width=6, height=6)
windows(record=TRUE, width=Width, height=Height)
}
resize.win(5,5)
plot(rnorm(100))
resize.win(10,10)
plot(rnorm(100))