Geohash approximates into a rectangle, so the actual, correct numbers are:
1 5,009.4km x 4,992.6km
2 1,252.3km x 624.1km
3 156.5km x 156km
4 39.1km x 19.5km
5 4.9km x 4.9km
6 1.2km x 609.4m
7 152.9m x 152.4m
8 38.2m x 19m
9 4.8m x 4.8m
10 1.2m x 59.5cm
11 14.9cm x 14.9cm
12 3.7cm x 1.9cm
courtesy of @odedfos: