Setting Active Layer Scale

As soon as you start a new document hold down the Control (ctrl) key at bottom left of your keyboard and click with the mouse to see the pop-up contextual menu, or just right-click if you have a two-button mouse.

Choose 'Active Layer Scale' and then choose a scale - 1:50 is a good choice for standard single rooms.

1:20 would be OK for small rooms... if you want a scale that is not a preset simple type the number into the blue highlighted box.

If you do not choose a scale the default is 1:1, which means that your A4 paper is smaller than a chair and about the thickness of a wall... to decide your scale imagine how many sheets of paper laid end-to-end you need to draw your plan - A4 is 297mm long by 210mm wide, so 1 metre is about 4 sheets long, 15 metres will be nearly 60 sheets long. You also need to leave enough space for your dimensions and any additional annotation.