I'm not sure which one is the scale size so i'll solve for both...
Ratio is 2cm:4m
0.02m:4m or 2cm:400cm also work as ratios
400cm/2cm=200 (so the scale is 200X smaller than the original)
Option 1::
If the scale of the item is:
l=12cm and w=6cm
then we can solve for the actual size by multiplying the values by 200
l=12cm*200=2400cm or 24m
w=6cm*200=1200cm or 12m
Option 2::
If the original item has a length of 12m and a width of 6m then:
l=12m and w=6m
We can find the scale length by dividing by 200
l=12m/200=.06m or 6cm
w=6m/200=0.03m or 3cm