You can add a line break to a title with \n and you can directly scale the vectors used in the plot.
plot(DF$A/1000 ~ DF$B, main = "Rather Long Title\nWith Line Break")
The aspect ratio of you plot looks unusual. Are you using the plotting pane of RStudio or something else to display the plot? There are commands to open a dedicated plot graphics window. On a Windows system you can use
windows(width = 9, height = 6)
and tune the width and height values. On Linux, I believe the function is x11().