I tend to do it using the edit land (4 metre square / multiply the number of sqaures by 4) method but there is another way to do this.
Stand on the corner of the plot, take a note of the co-ords in the viewer top bar and then walk to the next corner, take a note of the co-ords there and walk to the next corner, again take a note of the co-ords and so on.
You will end up with something like 0,16,20 & 16,16,21 for the first two, this means that the two corners are 16 metres apart (and the 2nd corner is one metre higher than the other).