Apricot,
When you click 'plot', you choose your printer and your paper size you want to print to. On the plot settings tab you will see 'printable area', that is the size you need to work with. Draw this size, and say, if you want it to print at 1 to 5, scale it up x5(make it five times bigger). When you plot, use the 'window' option on the 'plot area' and pick your rectangle. On the 'plot scale' you can specify 1 to 5 but it should still come out at 1 to 5 if you choose 'scaled to fit'.
Have fun,
Simon.