I propose you to draw a rectangle of 297x210 mm ( or 420x297 mm ) to send on your printer.

Your little printer has margins that will spoil the scale of your print but when you will launch the plot you will have the message in the printer dialog box :

 Plot scale : ( Apply Fit to paper ) 1 mm = 1.05 unit ( for example ).

If you do a copy/scale of your 297x210 / 1.05 = 282.857... x 200.000..., then when you print this rectangle of 282... x 200... the print will be at scale 1/1.

You can use this rectangle ( I use a double rectangle with the exact + little format together ) for a 1/1 scale.

You can do a scale by x 5 for a 1/5 print directly from the model space or use it at scale 1/1 in the layout space and open a viewport.

Please tell us if this helped or not.

Ask for more and have a good day.