So can I change the values of PrintableArea.Height and PrintableArea.Width?
You can't change the printable area size. This one is depending on the printer, the paper size and the orientation.
So you have to adapt the map size to the printable area size.
In this thread, you will find ideas on how to print the current map.
Another option is to clone the current map in order to keep live the current map and to display a print preview at the same time : sample.