I can't really see it, but is the scale is 1 to 50 divide the really length by 50 to get the measure measure of here drawing
so if the room is 10 by 20 meters
10 is 1000 cm
1000/50 = 20 cm
20 is 2000 cm
2000/50 = 40 cm
then the drawing of that room is 20cm by 40cm