..take those as a guideline and set them as 200 or 210 cm in length...
Or sometimes 190cm outside Holland/Scandinavia 
Hansmex' advice is good as there is usually some identifiable length somewhere in the drawing (pick the longest). Even a door/window could give an approximate result as long as the proportions are correct.
..a wild guess as nothing that allows an auto gauge even if i have the total area in sqft...
Actually, if you have the area, you might be able to make an estimation of the length of one wall. After you have the approximation (I would not call it a wild guess). You can go back and adjust. Create a room in the drawing and see how well it matches the area you have. Then you can change the measurements in the background drawing. When you start drawing a couple of walls, often the lengths will become apparent as there are often some standard measurements in a plan. ok
